feat(db/m7-353): add albums.cover_art_source enum column

internal/db/migrations/0016_album_cover_source.down.sql

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+DROP INDEX IF EXISTS albums_cover_art_source_idx;
+ALTER TABLE albums DROP CONSTRAINT IF EXISTS albums_cover_art_source_check;
+ALTER TABLE albums DROP COLUMN IF EXISTS cover_art_source;

internal/db/migrations/0016_album_cover_source.up.sql

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+-- M7 #353: album cover origin attribution.
+-- 'embedded'  — extracted from audio file (reserved for future slice).
+-- 'sidecar'   — found in the album directory (cover.jpg/folder.jpg/etc).
+-- 'mbcaa'     — fetched from MusicBrainz Cover Art Archive.
+-- 'none'      — tried, no cover available. Skip auto-retry; only manual.
+ALTER TABLE albums ADD COLUMN cover_art_source text;
+
+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'));
+
+-- Backfill existing rows with cover_art_path set (defensive: scanner has
+-- never populated cover_art_path in current dev, but keeps re-runs idempotent).
+UPDATE albums SET cover_art_source = 'sidecar'
+ WHERE cover_art_path IS NOT NULL AND cover_art_source IS NULL;
+
+-- Index supports "find missing covers" queries.
+CREATE INDEX albums_cover_art_source_idx ON albums (cover_art_source);