Add comprehensive test suite with CI workflow

- 211 tests across 15 files covering services (tracking, discovery,
  metadata, backup, credits, webhooks, settings, update-check, colors,
  person), utilities (config, cron, providers, title-theme), and TMDB
  image URL helpers
- `lib/test-preload.ts` + `bunfig.toml` wire up a global in-memory
  SQLite DB with migrations for all DB-backed tests
- `lib/test-utils.ts` provides `clearAllTables()` and seed helpers
  (insertUser, insertTitle, insertTvShow, insertMovieWatch, etc.)
- Export `getBackupSource`, `isKnownBackup`, `isValidBackupFilename`,
  `buildBackupCron`, `performUpdateCheck` internals for direct testing
- GitHub Actions workflow runs `bun test --coverage` on push/PR to main

lib/utils/title-theme.ts

@@ -1,6 +1,7 @@
 import type { ColorPalette } from "@/lib/types/title";
 
-function hexToRelativeLuminance(hex: string): number {
+/** @internal */
+export function hexToRelativeLuminance(hex: string): number {
   const r = Number.parseInt(hex.slice(1, 3), 16) / 255;
   const g = Number.parseInt(hex.slice(3, 5), 16) / 255;
   const b = Number.parseInt(hex.slice(5, 7), 16) / 255;