- Add `lib/constants.ts` exporting `DATA_DIR`, `DATABASE_URL`, `CACHE_DIR`,
`BACKUP_DIR`, `AVATAR_DIR`, `TMDB_API_BASE_URL`, and `TMDB_IMAGE_BASE_URL`
- Replace inline `process.env` derivations in `db/client`, `backup`,
`image-cache`, `system-health`, `tmdb/client`, `actions/settings`,
and `api/avatars` with imports from the new module
