fix(alerting/I-2): add unique partial index on alert_instances(rule_id) for open states

V13 migration creates alert_instances_open_rule_uq — a partial unique index on
(rule_id) WHERE state IN ('PENDING','FIRING','ACKNOWLEDGED'), preventing
duplicate open instances per rule. PostgresAlertInstanceRepository.save() catches
DuplicateKeyException and returns the existing open instance instead of failing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cameleer-server-app/src/main/resources/db/migration/V13__alert_instances_open_unique.sql

@@ -0,0 +1,7 @@
+-- V13 — Unique partial index: at most one open alert_instance per rule
+-- Prevents duplicate FIRING rows in multi-replica deployments.
+-- The Java save() path catches DuplicateKeyException and log-and-skips the losing insert.
+CREATE UNIQUE INDEX alert_instances_open_rule_uq
+  ON alert_instances (rule_id)
+  WHERE rule_id IS NOT NULL
+    AND state IN ('PENDING','FIRING','ACKNOWLEDGED');