fix: address code review findings

- Fix notifications_sent schema: change to composite PK (rgid, sent_at) per plan spec, remove FK constraint
- Change MarkNotificationSent from INSERT OR REPLACE to INSERT (composite PK semantics)
- Update test: replace idempotent test with duplicate-second and different-time tests
- Refactor UpdateArtistSettings to use switch-based column validation instead of fmt.Sprintf with map lookup
- Remove generated coverage.out from repo, add to .gitignore

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/database/notifications_test.go

@@ -41,8 +41,10 @@ func TestMarkNotificationSent_New(t *testing.T) {
 	}
 }
 
-// TestMarkNotificationSent_Idempotent verifies that marking the same RGID twice does not fail.
-func TestMarkNotificationSent_Idempotent(t *testing.T) {
+// TestMarkNotificationSent_DuplicateSecond verifies that inserting the same RGID twice
+// within the same second fails due to the composite primary key (rgid, sent_at).
+// In practice, notifications are sent at most once per day, so this is acceptable.
+func TestMarkNotificationSent_DuplicateSecond(t *testing.T) {
 	db, err := New(":memory:")
 	if err != nil {
 		t.Fatalf("New() error: %v", err)
@@ -58,8 +60,10 @@ func TestMarkNotificationSent_Idempotent(t *testing.T) {
 	if err := MarkNotificationSent(db, "rgid-1"); err != nil {
 		t.Fatalf("first MarkNotificationSent() error: %v", err)
 	}
-	if err := MarkNotificationSent(db, "rgid-1"); err != nil {
-		t.Fatalf("second MarkNotificationSent() error: %v", err)
+	// Second insert in the same second should fail with a UNIQUE constraint error.
+	err = MarkNotificationSent(db, "rgid-1")
+	if err == nil {
+		t.Fatal("expected UNIQUE constraint error on duplicate insert, got nil")
 	}
 
 	// Should still have exactly one row.
@@ -73,6 +77,52 @@ func TestMarkNotificationSent_Idempotent(t *testing.T) {
 	}
 }
 
+// TestMarkNotificationSent_DifferentTime verifies that inserting the same RGID at a
+// different explicit sent_at time succeeds (composite PK allows multiple rows per RGID).
+func TestMarkNotificationSent_DifferentTime(t *testing.T) {
+	db, err := New(":memory:")
+	if err != nil {
+		t.Fatalf("New() error: %v", err)
+	}
+	defer db.Close()
+
+	if err := insertTestArtist(db, "artist-1"); err != nil {
+		t.Fatalf("insertTestArtist: %v", err)
+	}
+	if err := insertTestRelease(db, "rgid-1", "artist-1"); err != nil {
+		t.Fatalf("insertTestRelease: %v", err)
+	}
+
+	// Insert with explicit different timestamps.
+	_, err = db.Conn().Exec("INSERT INTO notifications_sent (rgid, sent_at) VALUES (?, ?)", "rgid-1", "2024-01-01T00:00:00Z")
+	if err != nil {
+		t.Fatalf("first insert error: %v", err)
+	}
+	_, err = db.Conn().Exec("INSERT INTO notifications_sent (rgid, sent_at) VALUES (?, ?)", "rgid-1", "2024-06-01T00:00:00Z")
+	if err != nil {
+		t.Fatalf("second insert error: %v", err)
+	}
+
+	// IsNotificationSent should return true.
+	sent, err := IsNotificationSent(db, "rgid-1")
+	if err != nil {
+		t.Fatalf("IsNotificationSent() error: %v", err)
+	}
+	if !sent {
+		t.Error("expected IsNotificationSent to return true")
+	}
+
+	// Should have two rows.
+	var count int
+	err = db.Conn().QueryRow("SELECT COUNT(*) FROM notifications_sent WHERE rgid = ?", "rgid-1").Scan(&count)
+	if err != nil {
+		t.Fatalf("count query error: %v", err)
+	}
+	if count != 2 {
+		t.Errorf("expected 2 notification rows, got %d", count)
+	}
+}
+
 // TestIsNotificationSent_True verifies true for a sent notification.
 func TestIsNotificationSent_True(t *testing.T) {
 	db, err := New(":memory:")