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/database.go

@@ -96,8 +96,9 @@ func (db *DB) migrate() error {
 		{
 			name: "003_create_notifications_sent",
 			sql: `CREATE TABLE IF NOT EXISTS notifications_sent (
-				rgid TEXT PRIMARY KEY REFERENCES external_releases(rgid),
-				sent_at DATETIME DEFAULT CURRENT_TIMESTAMP
+				rgid TEXT NOT NULL,
+				sent_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+				PRIMARY KEY (rgid, sent_at)
 			);`,
 		},
 	}