Switch persistence of media file identifiers from a JSON file to a
SQLite database. Introduce DB_PATH, init_db_sync, upsert, update, and
removal functions. Remove json-repair dependency and related locking
logic. Update api_server and post_parser to use the new database
helpers.
BREAKING CHANGE: media_file_ids.json is no longer used; existing
data must be migrated to the new SQLite database.