Existing installs hold their whole library in jellyfin.db, so switching to the
PostgreSQL provider needs a one-shot data move.
- add Jellyfin.DbMigrator reading SQLite tables and bulk-writing them to PostgreSQL
- validate table names against the source schema before generating SQL
- upload a pre-migration copy of the SQLite file to S3 when S3_BACKUP_BUCKET is set
- emit a per-table row-count report and support a dry-run mode
