chore(server/m7-coverage-gauge): align coverage test with peer style

Code-review polish on admin_coverage_test.go:
- Replace inline artist-seed SQL with the seedArtist helper used
  across the rest of the api test suite (admin_covers_test.go,
  likes_test.go, me_history_test.go, admin_quarantine_test.go).
- Rename the cover_art_source pointer locals from none/sidecar/mbcaa
  to sourceNone/sourceSidecar/sourceMbcaa so they don't read like
  zero-value identifiers.

internal/api/admin_coverage_test.go

@@ -56,13 +56,7 @@ func TestAdminLibraryCoverage_MixedRowsReturnCorrectBuckets(t *testing.T) {
 	admin := seedUser(t, pool, "covgauge2", "pw", true)
 
 	// Seed an artist for FK satisfaction.
-	var artistID string
+	artist := seedArtist(t, pool, "Coverage Test")
-	if err := pool.QueryRow(context.Background(), `
-		INSERT INTO artists (name, sort_name) VALUES ('Coverage Test', 'coverage test')
-		RETURNING id
-	`).Scan(&artistID); err != nil {
-		t.Fatalf("seed artist: %v", err)
-	}
 
 	// Seed albums covering each bucket. Use distinct titles to avoid the
 	// unique (artist_id, title) constraint. Distinct mbids where set so
@@ -72,24 +66,24 @@ func TestAdminLibraryCoverage_MixedRowsReturnCorrectBuckets(t *testing.T) {
 		source *string
 		mbid   *string
 	}
-	none := "none"
+	sourceNone := "none"
-	sidecar := "sidecar"
+	sourceSidecar := "sidecar"
-	mbcaa := "mbcaa"
+	sourceMbcaa := "mbcaa"
 	mbid1 := "11111111-1111-1111-1111-111111111111"
 	mbid2 := "22222222-2222-2222-2222-222222222222"
 	mbid3 := "33333333-3333-3333-3333-333333333333"
 	rows := []seed{
-		{title: "WithArtSidecar", source: &sidecar, mbid: &mbid1}, // with_art
+		{title: "WithArtSidecar", source: &sourceSidecar, mbid: &mbid1}, // with_art
-		{title: "WithArtMbcaa", source: &mbcaa, mbid: &mbid2},     // with_art
+		{title: "WithArtMbcaa", source: &sourceMbcaa, mbid: &mbid2},     // with_art
 		{title: "PendingHasMbid", source: nil, mbid: &mbid3},            // pending (eligible)
 		{title: "PendingNoMbid", source: nil, mbid: nil},                // pending + pending_no_mbid
-		{title: "Settled", source: &none, mbid: nil},              // settled (no mbid is fine here)
+		{title: "Settled", source: &sourceNone, mbid: nil},              // settled (no mbid is fine here)
 	}
 	for _, s := range rows {
 		if _, err := pool.Exec(context.Background(), `
 			INSERT INTO albums (artist_id, title, sort_title, cover_art_source, mbid)
 			VALUES ($1, $2, $3, $4, $5)
-		`, artistID, s.title, s.title, s.source, s.mbid); err != nil {
+		`, artist.ID, s.title, s.title, s.source, s.mbid); err != nil {
 			t.Fatalf("seed album %q: %v", s.title, err)
 		}
 	}