feat(search): gate index events by active provider and harden rebuild reconcile

Completes the search-provider-interface wiring that the catalog hardening
commits already call into:

- Skip the transactional search-index-event write path when Meilisearch is
  not the active provider (ItemRepository.WithActiveSearchProvider /
  SearchIndexEventRepository.disabledByActiveProvider).
- Dead-letter catalog_search_index_events after 10 attempts instead of
  retrying forever.
- Track the rebuild high-water mark (MaxEventID / MarkProcessedThrough) and
  persist last_processed_event_id in UpdateStateAfterRebuild so a rebuild
  reconciles events enqueued during the rebuild.
- Validate (read-only) the embedding lock when embedding a search query
  instead of establishing/mutating it.
- Surface total_exact on the legacy /items browse response.
- Wire the active catalog search provider into the scanner and item repo at
  startup.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

cmd/silo/main.go

@@ -520,6 +520,12 @@ func main() {
 	// still encrypts/decrypts — no raw settings repo may escape into later wiring.
 	settingsRepo = catalog.NewEncryptedSettingsRepo(catalog.NewServerSettingsRepo(pool), dataCipher)
 	nodeID := resolveNodeIdentity()
+	catalogSearchStartupSettings, err := catalog.CatalogSearchSettingsFromMap(settings)
+	if err != nil {
+		slog.Warn("catalog search: failed to load settings for startup wiring; using postgres", "err", err)
+		catalogSearchStartupSettings = catalog.DefaultCatalogSearchSettings()
+	}
+	activeCatalogSearchProvider := catalog.ActiveCatalogSearchProvider(catalogSearchStartupSettings)
 
 	// Step 9: Validate
 	if err := cfg.Validate(); err != nil {
@@ -818,6 +824,7 @@ func main() {
 
 		ffprobePath := scanner.FFprobePathFromFFmpeg(cfg.Playback.FFmpegPath)
 		s := scanner.NewScanner(fileRepo, ffprobePath, deps.S3Public, cfg.Scanner.Workers, cfg.Scanner.EmptyTrashAfterScan)
+		s.SetSearchIndexProvider(activeCatalogSearchProvider)
 		configWatcher.OnChange(func(_, updated *config.Config) {
 			s.SetWorkers(updated.Scanner.Workers)
 		})
@@ -1064,7 +1071,7 @@ func main() {
 	if needsWorkers && deps.DB != nil && deps.FileRepo != nil {
 		chainRepo := metadata.NewChainRepository(deps.DB)
 		skippedRootRepo = metadata.NewSkippedRootRepository(deps.DB)
-		itemRepo = catalog.NewItemRepository(deps.DB)
+		itemRepo = catalog.NewItemRepository(deps.DB).WithActiveSearchProvider(activeCatalogSearchProvider)
 		episodeRepo = catalog.NewEpisodeRepository(deps.DB)
 		seasonRepo = catalog.NewSeasonRepository(deps.DB)
 		personRepo := catalog.NewPersonRepository(deps.DB)