feat(scanner): extractor-version (ev) mechanism — parser fixes reach old rows without a force rescan (faceted plan Phase 10)

Parser fixes (7e4c97f6, genre model v2) never reached rows ingested
under an older parser: the sidecar never changed, so every fingerprint
said 'unchanged'. The manifest JSONB now carries the reserved 'ev'
stamp — the parser version that produced the item's row.

Semantics (all pinned by tests):
- ABSENT ev decodes as 1 ('v1-or-unknown'); stamping is truthful-only:
  create paths + post-extraction stores stamp the setting's current
  value; deploy-day backfill and the extract-failure guard stay UNSET
  (a row never claims a version its data can't prove — a mid-backfill
  bump self-heals next poll); repoint reconciliation preserves the
  stored ev (stat-only refresh re-extracted nothing).
- NEW setting scan_extractor_version (seeded + registry + getter,
  int default 1, category scanner): a manifest whose ev != the setting
  re-extracts through the SAME change path (full extraction →
  updateMediaItemWithMetadata → fresh stamped manifest). Sidecar-less
  books get an entries-less manifest as their ev home (create: stamped;
  first-sighting: unset).
- Catch-up bounded: evReextractBudgetPerScan=100, atomic (watcher pass
  can overlap a poll scan), reset per ScanFolders — amortized +
  resumable across polls; no force rescan, no flood.
- Multifile books included: the ev gate rides multifileSidecarChanged's
  shared ladder; ev-driven manifestChanged drives the editorial apply
  + stamped re-store.

Settings API: the unified GET/PUT /api/system/settings now lists the
key (registry-driven, auto-surfaces in the admin UI). Test harness:
the registry is now Loaded + wired into the SystemSettingsHandler
(main.go parity) — settings writes through the API were 503s before.

Verified: unit (ev normalization, budget CAS, GetExtractorVersion
cache paths, registry clamping); integration state machine —
create-stamp ev=1 → equality skips (corruption probe persists) →
wiped scan-state backfills UNSET without re-extract → bump re-extracts
ONCE (sidecar value restored, ev=2) → stable (second corruption
persists); multifile twin green; adjacent suites (Audiobook, Calibre,
CrossLibraryMove, SchemaIdempotency) green; live app restart re-scans
1198 files with 0 changed (deploy-day rule), setting listed at
/api/system/settings.

internal/services/media_scanner_audio_multifile.go

@@ -1053,6 +1053,11 @@ func (s *MediaScanner) seedMultiFileManifest(ctx context.Context, libraryID pgty
 	s.scanStateUpsert(libraryID, newEntry)
 	if computed := statSidecarManifest(representative, dir); len(computed.Entries) > 0 {
 		s.storeManifest(ctx, libraryID, newEntry, hashSidecarManifest(representative, dir, computed))
+	} else {
+		// Sidecar-less book still needs an ev home (faceted plan Phase
+		// 10): stamp — the book was JUST extracted by the current
+		// parser, so the stamp is truthful.
+		s.storeManifest(ctx, libraryID, newEntry, &sidecarManifest{Version: sidecarManifestVersion})
 	}
 }