feat(db/m7-cover-sources): SQL queries for provider settings + recheck

- Extends GetAlbumCoverageRollup's with_art FILTER to include
  'theaudiodb' so F's coverage gauge surfaces TheAudioDB-found rows
  as with_art rather than silently undercounting.
- Modifies ListAlbumsMissingCover to take the current sources_version
  as a parameter; eligibility now includes stale-'none' rows.
- Adds SetAlbumCoverWithVersion (atomic write of path + source +
  version stamp) and the parallel SetArtistArtWithVersion +
  ClearArtistArtNone + ListArtistsMissingArt for the artist-art
  enricher.
- New coverart_settings.sql with ListProviderSettings,
  UpsertProviderSettings (boot reconciliation, preserves operator
  values on conflict), UpdateProviderSettings (admin PATCH; tri-
  state api_key handling), GetCurrentSourcesVersion,
  BumpSourcesVersion (RETURNING the new value).

internal/db/queries/artists.sql

@@ -11,7 +11,15 @@ DO UPDATE SET
 RETURNING *;
 
 -- name: GetArtistByID :one
-SELECT * FROM artists WHERE id = $1;
+-- M7 cover-sources: enricher reads the artist's mbid + current art
+-- columns to drive eligibility check. (Modified from SELECT * to
+-- explicitly include the new artist_art_* columns added in migration
+-- 0018.)
+SELECT a.id, a.name, a.sort_name, a.mbid,
+       a.artist_thumb_path, a.artist_fanart_path,
+       a.artist_art_source, a.artist_art_sources_version
+  FROM artists a
+ WHERE a.id = $1;
 
 -- name: GetArtistByName :one
 SELECT * FROM artists WHERE name = $1 LIMIT 1;
@@ -83,3 +91,38 @@ UPDATE artists
    SET mbid = $2,
        updated_at = now()
  WHERE id = $1 AND mbid IS NULL;
+
+-- name: ListArtistsMissingArt :many
+-- M7 cover-sources: returns artist rows the artist-art enricher should
+-- attempt. Eligible: artist_art_source IS NULL OR (= 'none' AND stale
+-- artist_art_sources_version). MBID-less artists are returned but the
+-- enricher skips them at the row.mbid == nil guard inside the chain.
+SELECT a.id
+  FROM artists a
+ WHERE a.artist_art_source IS NULL
+    OR (a.artist_art_source = 'none' AND a.artist_art_sources_version != $1)
+ ORDER BY a.created_at
+ LIMIT $2;
+
+-- name: SetArtistArtWithVersion :exec
+-- M7 cover-sources: records a successful or settled artist-art
+-- enrichment with the current sources_version stamp. thumb_path /
+-- fanart_path are NULL on a 'none' result, or independently NULL if
+-- the provider returned only one of the two image types (e.g.
+-- TheAudioDB had a thumb but no fanart for this artist).
+UPDATE artists
+   SET artist_thumb_path           = $2,
+       artist_fanart_path          = $3,
+       artist_art_source           = $4,
+       artist_art_sources_version  = $5,
+       updated_at                  = now()
+ WHERE id = $1;
+
+-- name: ClearArtistArtNone :exec
+-- M7 cover-sources: parallel of ClearAlbumCoverNone. Used by the
+-- eligibility loop to flip stale 'none' rows back to NULL.
+UPDATE artists
+   SET artist_art_source = NULL,
+       updated_at        = now()
+ WHERE id = $1
+   AND artist_art_source = 'none';