v0.3.1: per-ISO boot password gate

Operators can now lock individual ISOs behind a password set in the
WebUI. Picking a locked image at the PXE menu prompts the operator on
the client console; the boot script is only released after a correct
match. The plaintext never leaves the request — server stores bcrypt
hashes, scripts never echo the candidate.

## Backend

- New optional `password_hash: Option<String>` on `IsoMeta`. Skipped
  during serialize when None, so existing meta.json files don't grow
  a noisy `null` field.
- `IsoStore::set_password(id, Some("pw"))` hashes via bcrypt
  `DEFAULT_COST` (10 — fast enough for an interactive iPXE prompt,
  expensive enough to be hostile to brute force on a leaked
  meta.json). `set_password(id, None)` and `set_password(id, Some(""))`
  both clear.
- `IsoStore::verify_password` returns Ok(true) when no password is
  set, so the gate stays open for the common case.
- `IsoMeta::is_password_protected()` predicate the HTTP layer + UI
  share.
- NFS-sourced ISOs persist their hash in memory only — the share is
  the source of truth for those, and it doesn't carry hash sidecars.

## HTTP API

- `PUT /api/isos/:id/password` body `{ "password": "..." }` to set,
  `{ "password": null }` (or empty string) to clear.
- `DELETE /api/isos/:id/password` for the explicit clear.
- Both 204 on success, 404 for unknown ids.
- `/boot/<entry>.ipxe` now intercepts:
  - no `?token=`        -> render password-prompt script
  - `?token=<wrong>`    -> render auth-fail script (sleeps 2s, chains
                           back to the entry which re-prompts)
  - `?token=<correct>`  -> render the real boot script
  - ISO without password ignores token entirely (per-MAC bookmarks
    still work without changes).

## iPXE prompt

`render_password_prompt`:
- `set password ` then `read --secret password` — accepts input
  without echoing.
- Empty input chains back to the main menu (lets the operator back
  out of a misclick).
- Submit chains `?token=${password:uristring}`. The `:uristring`
  modifier URL-encodes the value, so passwords with `&`, `?`, `=`,
  spaces, etc. survive transport.

`render_password_failed`:
- Single line saying so + 2s sleep, then re-chains the entry.
- Server-side WARN log records the entry id only, never the
  candidate value (verified in smoke test).

## UI

Storage tab's image table grows an `Auth` column showing
`protected` / `open`, plus a 🔒 next to the filename when locked.
Per-row "Set password" / "Password ✎" button toggles an inline
editor in the next table row containing:
- a "Password protect this image" checkbox
- a `<input type=password autocomplete=new-password>` (hidden when
  the checkbox is off)
- a Save button

Save calls PUT or DELETE on `/api/isos/:id/password` based on the
checkbox state and clears the input field before re-rendering, so
the plaintext doesn't sit in the DOM longer than needed.

## Menu indicator

`render_family_menu` adds a `*` prefix immediately before the size
box on protected entries — ASCII only because some firmware menu
consoles mangle non-ASCII glyphs. Looks like:

  item --key 1 win11_test-winpe *[ 5234 MB] Windows 11 Test ISO

## Tests

74 passing across the workspace (was 66 in v0.3.0):
- 3 new store unit tests (bcrypt round-trip, unknown-id error,
  meta.json persistence across restart)
- 2 new ipxe_script unit tests (prompt/auth-fail invariants:
  read --secret, uristring, no candidate echo)
- 3 new HTTP integration tests (full gate flow upload-set-prompt-
  fail-success-clear, null/empty bodies, 404 on unknown id)

cargo clippy --workspace --all-targets clean.

Local smoke verified upload + lock + prompt + auth-fail + correct +
menu indicator + log scrub on a real release binary.

## Operational notes

- HTTP, not HTTPS — token rides in the query string. Acceptable on
  a trusted boot VLAN; do NOT expose OpenPXE to untrusted networks
  with this feature relied on for security. Reverse-proxy in front
  of OpenPXE will end up with the token in access logs.
- bcrypt cost is `DEFAULT_COST` (10). One verify takes ~50ms on
  modern x86, which is the worst-case latency added to a correct
  boot. Tunable via the bcrypt crate if needed.

crates/iso-store/src/store.rs

@@ -48,6 +48,25 @@ pub struct IsoMeta {
     /// Old `meta.json` files without this field deserialize as `Local`.
     #[serde(default)]
     pub source: IsoSource,
+    /// Optional bcrypt hash of an operator-set password. When present,
+    /// `/boot/<entry>.ipxe` returns a `read --secret` prompt instead of
+    /// the boot script until the client chains back with the correct
+    /// `?token=...`. We never store, log, or transmit the plaintext.
+    /// Skipped on serialize when None to keep meta.json clean for
+    /// the common no-password case.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub password_hash: Option<String>,
+}
+
+impl IsoMeta {
+    /// Convenience predicate the HTTP layer + UI can both use.
+    #[must_use]
+    pub fn is_password_protected(&self) -> bool {
+        self.password_hash
+            .as_deref()
+            .map(str::trim)
+            .is_some_and(|h| !h.is_empty())
+    }
 }
 
 pub struct UploadHandle {
@@ -94,6 +113,7 @@ impl UploadHandle {
             introspection,
             boot_entries,
             source: IsoSource::Local,
+            password_hash: None,
         };
         store.persist_meta(&meta).await?;
         store.insert(meta.clone());
@@ -289,6 +309,7 @@ impl IsoStore {
             introspection,
             boot_entries,
             source,
+            password_hash: None,
         };
         self.inner.write().isos.insert(id, meta);
     }
@@ -302,6 +323,66 @@ impl IsoStore {
             !matches!(&m.source, IsoSource::Nfs { mount_id: mid, .. } if mid == mount_id)
         });
     }
+
+    /// Set or clear an ISO's boot password.
+    ///
+    /// `Some("plaintext")` hashes via bcrypt (cost 10 — fast enough for
+    /// an interactive iPXE prompt, slow enough to be hostile to brute
+    /// force on a leaked meta.json) and persists.
+    ///
+    /// `None` removes the password — the next /boot/<id>.ipxe request
+    /// returns the script directly without a prompt.
+    ///
+    /// We never store, log, or transmit the plaintext.
+    pub async fn set_password(&self, id: &str, password: Option<&str>) -> Result<()> {
+        let new_hash = match password {
+            None => None,
+            Some(pw) => {
+                let pw = pw.trim();
+                if pw.is_empty() {
+                    None
+                } else {
+                    let h = bcrypt::hash(pw, bcrypt::DEFAULT_COST)
+                        .map_err(|e| Error::Other(e.into()))?;
+                    Some(h)
+                }
+            }
+        };
+
+        // Update in-memory + grab a clone for persistence outside the lock.
+        let updated = {
+            let mut g = self.inner.write();
+            let m = g
+                .isos
+                .get_mut(id)
+                .ok_or_else(|| Error::Invalid(format!("no such iso '{id}'")))?;
+            m.password_hash = new_hash;
+            m.clone()
+        };
+        // NFS-sourced ISOs have no on-disk meta.json — skip persistence
+        // for them (the password lives in memory until the manager
+        // re-scans the share, then it's gone). Document this in the API
+        // handler so the operator knows.
+        if matches!(updated.source, IsoSource::Local) {
+            self.persist_meta(&updated).await?;
+        }
+        Ok(())
+    }
+
+    /// Verify a candidate password against the stored bcrypt hash.
+    /// Returns:
+    /// - `Ok(true)`   — match (or the ISO has no password set; gate is open)
+    /// - `Ok(false)`  — mismatch
+    /// - `Err(_)`     — id not found, or bcrypt error
+    pub fn verify_password(&self, id: &str, candidate: &str) -> Result<bool> {
+        let meta = self
+            .get(id)
+            .ok_or_else(|| Error::Invalid(format!("no such iso '{id}'")))?;
+        let Some(hash) = meta.password_hash else {
+            return Ok(true); // no password set — anyone can boot
+        };
+        bcrypt::verify(candidate, &hash).map_err(|e| Error::Other(e.into()))
+    }
 }
 
 fn slugify(filename: &str) -> String {
@@ -416,6 +497,8 @@ fn linux_cmdline(family: DistroFamily, id: &str) -> String {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::introspect::{DistroFamily, IntrospectionReport};
+    use tempfile::tempdir;
 
     #[test]
     fn slugify_basic() {
@@ -427,4 +510,83 @@ mod tests {
         // If a path sneaks in, file_stem strips the directory — OK, not a hazard.
         assert_eq!(slugify("/etc/passwd"), "passwd");
     }
+
+    fn fake_meta(id: &str) -> IsoMeta {
+        IsoMeta {
+            id: id.into(),
+            filename: format!("{id}.iso"),
+            size_bytes: 0,
+            sha256_hex: None,
+            uploaded_at: OffsetDateTime::now_utc(),
+            introspection: IntrospectionReport {
+                family: DistroFamily::Unknown,
+                volume_label: None,
+                kernel_path: None,
+                initrd_paths: vec![],
+                has_boot_wim: false,
+            },
+            boot_entries: vec![],
+            source: IsoSource::Local,
+            password_hash: None,
+        }
+    }
+
+    #[tokio::test]
+    async fn password_round_trip_set_verify_clear() {
+        let dir = tempdir().unwrap();
+        let store = IsoStore::new(dir.path().to_path_buf());
+        store.ensure_dirs().await.unwrap();
+        store.inner.write().isos.insert("alpha".into(), fake_meta("alpha"));
+
+        // No password set — verify_password returns Ok(true) for any input.
+        assert!(store.verify_password("alpha", "anything").unwrap());
+        assert!(!store.get("alpha").unwrap().is_password_protected());
+
+        // Set a password.
+        store.set_password("alpha", Some("hunter2")).await.unwrap();
+        let m = store.get("alpha").unwrap();
+        assert!(m.is_password_protected());
+        assert!(m.password_hash.unwrap().starts_with("$2"));
+
+        // Verify correct + wrong.
+        assert!(store.verify_password("alpha", "hunter2").unwrap());
+        assert!(!store.verify_password("alpha", "wrong").unwrap());
+        assert!(!store.verify_password("alpha", "").unwrap());
+
+        // Clear by passing None or an empty string.
+        store.set_password("alpha", None).await.unwrap();
+        assert!(!store.get("alpha").unwrap().is_password_protected());
+        store.set_password("alpha", Some("again")).await.unwrap();
+        store.set_password("alpha", Some("   ")).await.unwrap();
+        assert!(!store.get("alpha").unwrap().is_password_protected());
+    }
+
+    #[tokio::test]
+    async fn set_password_for_unknown_id_errors() {
+        let dir = tempdir().unwrap();
+        let store = IsoStore::new(dir.path().to_path_buf());
+        store.ensure_dirs().await.unwrap();
+        let r = store.set_password("does-not-exist", Some("pw")).await;
+        assert!(matches!(r, Err(Error::Invalid(_))));
+    }
+
+    #[tokio::test]
+    async fn password_persists_via_meta_json_for_local_isos() {
+        // Hash makes it onto disk so it survives a restart.
+        let dir = tempdir().unwrap();
+        let store = IsoStore::new(dir.path().to_path_buf());
+        store.ensure_dirs().await.unwrap();
+        let meta = fake_meta("alpha");
+        store.persist_meta(&meta).await.unwrap();
+        store.insert(meta);
+        store.set_password("alpha", Some("s3cret")).await.unwrap();
+
+        // Re-load from disk and confirm the hash came back.
+        let store2 = IsoStore::new(dir.path().to_path_buf());
+        store2.load_from_disk().await.unwrap();
+        let reloaded = store2.get("alpha").expect("reloaded");
+        assert!(reloaded.is_password_protected());
+        assert!(store2.verify_password("alpha", "s3cret").unwrap());
+        assert!(!store2.verify_password("alpha", "wrong").unwrap());
+    }
 }