AlertNotificationRepository gains resetForRetry(UUID, Instant) which sets
attempts=0, status=PENDING, next_attempt_at=now, and clears claim/response
fields. AlertNotificationController calls resetForRetry instead of
scheduleRetry so a manual retry always starts from a clean slate.
AlertNotificationControllerIT adds retryResetsAttemptsToZero to verify
attempts==0 and status==PENDING after three prior markFailed calls.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>