Adds the schema for the pluggable cover-art provider abstraction:
- Extends albums.cover_art_source CHECK to accept 'theaudiodb' (the
constraint from migration 0016 only allowed embedded/sidecar/mbcaa/
none, which would block writes from the new provider).
- Adds artist_thumb_path / artist_fanart_path / artist_art_source
on artists, with a parallel CHECK constraint and source index.
- Adds *_sources_version stamps on both albums and artists for the
per-row recheck eligibility logic. Default 0 so every existing row
becomes stale relative to the seeded current_version=1, retrying
through the new chain on first scan after migrate.
- New cover_art_provider_settings table (per-provider enabled / api_key
/ display_order) and cover_art_sources_meta singleton holding
current_version. Seeds both v1 providers (mbcaa + theaudiodb) as
enabled.
- New artist_art_enrich jsonb column on scan_runs for the 4th
scan-orchestrator stage tally.
The seed leaves theaudiodb.api_key NULL; the application supplies the
upstream's documented test key (2) as the default when the column is
NULL, per the no-coercive-settings principle.
