Add a new `mime_type` column to the media_file_ids table and provide
functions to get and set cached MIME types. The API now attempts to
retrieve the MIME type from the database before falling back to
python-magic, persisting the detected type for future requests. This
reduces I/O overhead and improves response performance. Also adjust
logging to debug level to reduce noise.
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.