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  matching.go MatchingHandler twins), superseded helpers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/database/settings_registry.go

@@ -24,9 +24,9 @@ import (
 
 // SettingType enumerates the value types stored in system_settings.setting_type.
 const (
-	SettingTypeInt       = "int"
-	SettingTypeBool      = "bool"
-	SettingTypeString    = "string"
+	SettingTypeInt        = "int"
+	SettingTypeBool       = "bool"
+	SettingTypeString     = "string"
 	SettingTypeStringList = "string_list"
 )
 
@@ -194,7 +194,9 @@ func (r *SettingsRegistry) AutoScanEnabled() bool { return r.getBool("auto_scan_
 // AutoPlaylistCollections reports whether the scanner should turn
 // obviously-playlist sidecar series ("The Witcher (Reading Order)") into
 // curated static collections (A2c §2.4).
-func (r *SettingsRegistry) AutoPlaylistCollections() bool { return r.getBool("auto_playlist_collections") }
+func (r *SettingsRegistry) AutoPlaylistCollections() bool {
+	return r.getBool("auto_playlist_collections")
+}
 
 // ScanExtractorVersion is the parser version stamped on each book's scan
 // manifest. Books whose stored stamp differs re-extract their metadata on
@@ -298,7 +300,7 @@ func (r *SettingsRegistry) SyncQueueConfig() SyncQueueConfig {
 // WorkerPoolConfig bundles worker count and queue capacity. Read at startup;
 // changes require a restart.
 type WorkerPoolConfig struct {
-	Size   int
+	Size     int
 	QueueCap int
 }