fix(alerting/I-1): retry endpoint resets attempts to 0 instead of incrementing

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>

cameleer-server-core/src/main/java/com/cameleer/server/core/alerting/AlertNotificationRepository.java

@@ -13,5 +13,7 @@ public interface AlertNotificationRepository {
     void markDelivered(UUID id, int status, String snippet, Instant when);
     void scheduleRetry(UUID id, Instant nextAttemptAt, int status, String snippet);
     void markFailed(UUID id, int status, String snippet);
+    /** Resets a FAILED notification for operator-triggered retry: attempts → 0, status → PENDING. */
+    void resetForRetry(UUID id, Instant nextAttemptAt);
     void deleteSettledBefore(Instant cutoff);
 }