feat(db/m7-cover-sources): migration 0018 — artist art + provider settings

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.

internal/db/migrations/0018_artist_art_and_provider_settings.down.sql

@@ -0,0 +1,26 @@
+-- Reverse of 0018.
+
+ALTER TABLE scan_runs DROP COLUMN IF EXISTS artist_art_enrich;
+
+DROP TABLE IF EXISTS cover_art_sources_meta;
+DROP TABLE IF EXISTS cover_art_provider_settings;
+
+ALTER TABLE artists DROP COLUMN IF EXISTS artist_art_sources_version;
+ALTER TABLE albums  DROP COLUMN IF EXISTS cover_art_sources_version;
+
+DROP INDEX IF EXISTS artists_artist_art_source_idx;
+ALTER TABLE artists DROP CONSTRAINT IF EXISTS artists_artist_art_source_check;
+ALTER TABLE artists DROP COLUMN IF EXISTS artist_art_source;
+ALTER TABLE artists DROP COLUMN IF EXISTS artist_fanart_path;
+ALTER TABLE artists DROP COLUMN IF EXISTS artist_thumb_path;
+
+-- Restore the original albums.cover_art_source CHECK from migration 0016.
+ALTER TABLE albums DROP CONSTRAINT IF EXISTS albums_cover_art_source_check;
+ALTER TABLE albums
+    ADD CONSTRAINT albums_cover_art_source_check
+    CHECK (cover_art_source IS NULL
+           OR cover_art_source IN ('embedded','sidecar','mbcaa','none'));
+-- Note: rows with cover_art_source='theaudiodb' will be rejected by this
+-- restored constraint. If down-migrating from a populated DB, run:
+--   UPDATE albums SET cover_art_source = NULL WHERE cover_art_source = 'theaudiodb';
+-- before applying this migration.

internal/db/migrations/0018_artist_art_and_provider_settings.up.sql

@@ -0,0 +1,62 @@
+-- M7 cover-sources: schema for additional cover-art providers (TheAudioDB
+-- as the v1 implementation), artist art (thumb + fanart), and the
+-- recheck-on-source-change trigger.
+
+-- (0) Extend the existing albums.cover_art_source CHECK to accept the
+-- new 'theaudiodb' value. The constraint was created in migration 0016
+-- with the IN list ('embedded','sidecar','mbcaa','none'); without this
+-- extension, the enricher's UPDATE setting cover_art_source='theaudiodb'
+-- would fail with a CHECK violation.
+ALTER TABLE albums DROP CONSTRAINT IF EXISTS albums_cover_art_source_check;
+ALTER TABLE albums
+    ADD CONSTRAINT albums_cover_art_source_check
+    CHECK (cover_art_source IS NULL
+           OR cover_art_source IN ('embedded','sidecar','mbcaa','theaudiodb','none'));
+
+-- (1) Artist art columns parallel albums.cover_art_*.
+ALTER TABLE artists
+    ADD COLUMN artist_thumb_path  text,
+    ADD COLUMN artist_fanart_path text,
+    ADD COLUMN artist_art_source  text;
+
+ALTER TABLE artists
+    ADD CONSTRAINT artists_artist_art_source_check
+    CHECK (artist_art_source IS NULL
+           OR artist_art_source IN ('theaudiodb','none'));
+
+CREATE INDEX artists_artist_art_source_idx ON artists (artist_art_source);
+
+-- (2) Recheck-trigger version stamps. Default 0; current_version starts
+-- at 1, so every existing row is "stale" relative to current — first
+-- scan after migrate retries every settled row through the new chain.
+ALTER TABLE albums
+    ADD COLUMN cover_art_sources_version int NOT NULL DEFAULT 0;
+ALTER TABLE artists
+    ADD COLUMN artist_art_sources_version int NOT NULL DEFAULT 0;
+
+-- (3) Per-provider settings + singleton meta row.
+CREATE TABLE cover_art_provider_settings (
+    provider_id   text PRIMARY KEY,
+    enabled       boolean NOT NULL DEFAULT true,
+    api_key       text,
+    display_order int  NOT NULL DEFAULT 0,
+    created_at    timestamptz NOT NULL DEFAULT now(),
+    updated_at    timestamptz NOT NULL DEFAULT now()
+);
+
+CREATE TABLE cover_art_sources_meta (
+    id              boolean PRIMARY KEY DEFAULT true,
+    current_version int NOT NULL DEFAULT 1,
+    CONSTRAINT cover_art_sources_meta_singleton CHECK (id = true)
+);
+INSERT INTO cover_art_sources_meta (id, current_version) VALUES (true, 1);
+
+-- Seed both v1 providers as enabled. TheAudioDB's api_key stays NULL;
+-- the application supplies the test key '2' when the column is NULL,
+-- per the no-coercive-settings principle.
+INSERT INTO cover_art_provider_settings (provider_id, enabled, display_order) VALUES
+    ('mbcaa',      true, 0),
+    ('theaudiodb', true, 1);
+
+-- (4) Scan-runs gains a stage tally column for artist art enrichment.
+ALTER TABLE scan_runs ADD COLUMN artist_art_enrich jsonb;