refactor: remove dead code across server, templ pages, and client modules

Deadcode-verified removals (RTA reachability from main, cross-checked
with -test and textual xrefs):

Go server:
- whole files: middleware/transaction.go, middleware/error_handler.go,
  middleware/password_validator.go (live registration machinery kept),
  handlers/sync.go, sync/offline.go
- unrouted endpoints: auth UpdateUsername/UpdateEmail, timezone
  settings handler, OPDS RegisterOPDS, Kobo LibrarySync handler,
  ReaderHandler.ShowReader, JSON-data twins on collections/library
- duplicate book-matching handlers (book_matching.go Handler twins,
  matching.go MatchingHandler twins), superseded helpers
  (router.parseUUID, handlers.ValidateDeviceToken, normalizePath,
  userExists, generateLockID)

Templ/client:
- never-routed pages: api_explorer.templ, queue.templ and their
  client modules api-explorer.ts, api-explorer-docs.ts, queue.ts
- fully orphaned wrapper modules events.ts, dom.ts
- broken clearSyncQueue (called a nonexistent endpoint) and the
  hidden, never-populated sync-queue section on the devices page
- 166+ unused exports demoted to private (knip-verified); ~85 truly
  dead declarations removed via tsc noUnusedLocals fixpoint
- stale 'interactive API Explorer' claims dropped from docs

The docs drift-gate package (internal/docs), templ-bound modules
(dashboard, stats, header, index, login, profile*, bookshelf,
collections, toast-error), sqlc-generated code, and web/static are
intentionally untouched.

internal/services/media_scanner.go

@@ -491,12 +491,6 @@ func (s *MediaScanner) SetTrigger(trigger string) {
 	s.scanTrigger = trigger
 }
 
-// GetChangedItems reports how many existing items re-processed this run
-// (size change, sidecar change, or force rescan).
-func (s *MediaScanner) GetChangedItems() int {
-	return s.changedItems
-}
-
 func (s *MediaScanner) GetStats() (int, int, int) {
 	return s.totalFiles, s.newItems, s.errors
 }
@@ -4272,16 +4266,6 @@ func (s *MediaScanner) scanStateUpsert(libraryID pgtype.UUID, entry *scanEntry)
 	s.scanState[scanStateKey(libraryID, entry.FilePath)] = entry
 }
 
-// scanStateRemove drops an entry (the row's file_path moved elsewhere).
-func (s *MediaScanner) scanStateRemove(libraryID pgtype.UUID, relPath string) {
-	s.scanStateMu.Lock()
-	defer s.scanStateMu.Unlock()
-	if s.scanState == nil {
-		return
-	}
-	delete(s.scanState, scanStateKey(libraryID, relPath))
-}
-
 func (s *MediaScanner) getMediaItemByFilePath(ctx context.Context, filePath string, libraryID pgtype.UUID) (database.MediaItems, error) {
 	return s.db.GetMediaItemByFilePath(ctx, database.GetMediaItemByFilePathParams{
 		FilePath:  s.getRelativePath(filePath),