feat(alerts): Postgres repo — read_at/deleted_at columns, filter params, new mutations

- save/rowMapper read+write read_at and deleted_at
- listForInbox: tri-state acked/read filters; always excludes deleted
- countUnreadBySeverity: rewire without alert_reads join, preserve zero-fill
- new: markRead/bulkMarkRead/softDelete/bulkSoftDelete/bulkAck/restore
- delete PostgresAlertReadRepository + its bean
- restore zero-fill Javadoc on interface
- mechanical compile-fixes in AlertController, InAppInboxQuery,
  AlertControllerIT, InAppInboxQueryTest; Task 6 owns the rewrite
- PostgresAlertReadRepositoryIT stubbed @Disabled; Task 7 owns migration

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -43,6 +43,8 @@ public interface AlertInstanceRepository {
      * Count unread alert instances visible to the user, grouped by severity.
      * Visibility: targets user directly, or via one of the given groups/roles.
      * "Unread" = {@code read_at IS NULL AND deleted_at IS NULL}.
+     * Always returns a map with an entry for every {@link AlertSeverity} (value 0 if no rows),
+     * so callers never need null-checks.
      */
     Map<AlertSeverity, Long> countUnreadBySeverity(UUID environmentId,
                                                     String userId,