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.go

@@ -7,7 +7,7 @@ import (
 // MarkNotificationSent records that a notification has been sent for the given RGID.
 func MarkNotificationSent(db *DB, rgid string) error {
 	_, err := db.Conn().Exec(
-		"INSERT OR REPLACE INTO notifications_sent (rgid) VALUES (?)",
+		"INSERT INTO notifications_sent (rgid) VALUES (?)",
 		rgid,
 	)
 	if err != nil {