v0.5.4: code-cleanup pass (AppError, figment config, encoding dedup, typed status, deps)

Final cleanup before hardware testing. No behaviour changes; 248 tests green,
clippy clean.

#1  AppError newtype (http-api/src/error.rs) with one IntoResponse mapping
    (NotFound→404, Invalid→400, _→500) + From<core::Error>/From<io::Error>.
    Converted the clearly-safe handlers (sso_put, unattended_upload,
    branding_clear) to `?`; intentionally left handlers with bespoke
    status semantics (Invalid→404 on category, 409 on duplicate share /
    open upload) explicit so no asserted status changes.
#2  figment-based Config::load (defaults → TOML → env). Keeps the historical
    flat OPENPXE_* names (Unraid/entrypoint compatible) AND adds the nested
    OPENPXE_SECTION__FIELD form; now covers every field (apply_env had
    silently skipped unattended_dir + bind addrs). 6 Jail tests prove
    backward-compat. Removed the hand-rolled apply_env.
#3  thiserror 1→2; dropped unused mime/mime_guess/once_cell deps.
#4  Re-evaluated: Duration::from_hours/from_mins are stable on the pinned
    1.95 toolchain and clippy prefers them — kept the readable form
    (the "unstable" premise didn't hold; MSRV is intentionally 1.95).
#5  insta snapshot of the rendered iPXE menu (version-filtered) + wiremock
    coverage of the SAML metadata-URL fetch (200 + non-2xx).
#6  api_status → typed StatusResponse struct (was a 25-key json! blob) with
    a full_flow guard test asserting every UI key + the started_at string
    shape. Deferred the /api/docs typed conversion (lowest value, highest
    churn, zero functional benefit).
#7  pct_encode/xml_escape de-duplicated into openpxe_core::encoding (were
    copied across app.rs + the SAML modules). No new crates.
#8  UploadSessions registry → parking_lot::RwLock (sync, never held across
    .await); per-session lock stays tokio::Mutex.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

crates/http-api/Cargo.toml

@@ -33,8 +33,6 @@ thiserror.workspace = true
 anyhow.workspace = true
 bytes.workspace = true
 futures.workspace = true
-mime.workspace = true
-mime_guess.workspace = true
 uuid.workspace = true
 # v0.4.5 Forms auth: lock-free session store and cookie helpers.
 parking_lot.workspace = true
@@ -61,3 +59,9 @@ image = { version = "0.25", default-features = false, features = ["png"] }
 # replay, and IdP-initiated-gating flows exercise real signatures.
 rcgen = "0.13"
 bergshamra = { workspace = true }
+# v0.5.4: snapshot the generated iPXE menu so any unintended drift (a
+# dropped line, reordered item) is caught and reviewed, not silently shipped.
+insta = "1.40"
+# v0.5.4: stand up a mock HTTP server to exercise the SAML metadata-URL
+# fetch path (previously untested because it did a real network GET).
+wiremock = "0.6"