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  matching.go MatchingHandler twins), superseded helpers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/schema.go

@@ -56,20 +56,10 @@ func init() {
 
 const (
 	// Pre-computed FNV-1a hash of "bookhoard:schema:init"
-	// Generated using: generateLockID("bookhoard:schema:init")
+	// (FNV offset basis 14695981039346656037, FNV prime 1099511628211)
 	schemaInitLockID = 7804706162000639061
 )
 
-// Hash calculation function (for reference/testing)
-func generateLockID(key string) int64 {
-	hash := uint64(14695981039346656037) // FNV offset basis
-	for _, c := range key {
-		hash ^= uint64(c)
-		hash *= 1099511628211 // FNV prime
-	}
-	return int64(hash)
-}
-
 // parseTableNames extracts all table names from CREATE TABLE statements
 // Uses stream scanning for performance and memory efficiency
 // Handles both "CREATE TABLE" and "CREATE TABLE IF NOT EXISTS" formats