feat(alerting): Postgres repositories for silences, notifications, reads
PostgresAlertSilenceRepository: save/findById roundtrip, listActive (BETWEEN
starts_at AND ends_at), listByEnvironment, delete. JSONB SilenceMatcher via ObjectMapper.
PostgresAlertNotificationRepository: save/findById, listForInstance,
claimDueNotifications (UPDATE...RETURNING with FOR UPDATE SKIP LOCKED),
markDelivered, scheduleRetry (bumps attempts + next_attempt_at), markFailed,
deleteSettledBefore (DELIVERED+FAILED rows older than cutoff). JSONB payload.
PostgresAlertReadRepository: markRead (ON CONFLICT DO NOTHING idempotent),
bulkMarkRead (iterates, handles empty list without error).
16 IT scenarios across 3 classes, all passing.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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