fix(alerting): allow multiple open alert_instances per rule for PER_EXCHANGE

V13 added a partial unique index on alert_instances(rule_id) WHERE state
IN (PENDING,FIRING,ACKNOWLEDGED). Correct for scalar condition kinds
(ROUTE_METRIC / AGENT_STATE / DEPLOYMENT_STATE / LOG_PATTERN / JVM_METRIC
/ EXCHANGE_MATCH in COUNT_IN_WINDOW) but wrong for EXCHANGE_MATCH /
PER_EXCHANGE, which by design emits one alert_instance per matching
exchange. Under V13 every PER_EXCHANGE tick with >1 match logged
"Skipped duplicate open alert_instance for rule …" at evaluator cadence
and silently lost alert fidelity — only the first matching exchange per
tick got an AlertInstance + webhook dispatch.

V15 drops the rule_id-only constraint and recreates it with a
discriminator on context->'exchange'->>'id'. Scalar kinds emit
Map.of() as context, so their expression resolves to '' — "one open per
rule" preserved. ExchangeMatchEvaluator.evaluatePerExchange always
populates exchange.id, so per-exchange instances coexist cleanly.

Two new PostgresAlertInstanceRepositoryIT tests:
  - multiple open instances for same rule + distinct exchanges all land
  - second open for identical (rule, exchange) still dedups via the
    DuplicateKeyException fallback in save() — defense-in-depth kept

Also fixes pre-existing PostgresAlertReadRepositoryIT brokenness: its
setup() inserted 3 open instances sharing one rule_id, which V13 blocked
on arrival. Migrate to one rule_id per instance (pattern already used
across other storage ITs).

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

cameleer-server-app/src/main/resources/db/migration/V15__alert_instances_open_rule_exchange_unique.sql

@@ -0,0 +1,24 @@
+-- V15 — Discriminate open-alert_instance uniqueness by exchange id for PER_EXCHANGE rules.
+--
+-- V13 enforced "one open alert_instance per rule" via a partial unique on
+-- (rule_id). That is correct for every scalar condition kind (ROUTE_METRIC,
+-- AGENT_STATE, DEPLOYMENT_STATE, LOG_PATTERN, JVM_METRIC, and EXCHANGE_MATCH
+-- in COUNT_IN_WINDOW mode) but wrong for EXCHANGE_MATCH / PER_EXCHANGE, which
+-- by design emits one alert_instance per matching exchange. Under V13 every
+-- PER_EXCHANGE tick with >1 match logged "Skipped duplicate open alert_instance
+-- for rule …" at evaluator cadence and silently lost alert fidelity — only the
+-- first matching exchange per tick got an AlertInstance + webhook dispatch.
+--
+-- New discriminator: (rule_id, COALESCE(context->'exchange'->>'id', '')).
+-- Scalar evaluators emit Map.of() (no exchange subtree), the expression
+-- resolves to '' for all of them, so they continue to get "one open per rule".
+-- ExchangeMatchEvaluator.evaluatePerExchange emits {exchange.id = <execId>}
+-- per firing, so PER_EXCHANGE instances for distinct exchanges coexist.
+-- Two attempts to open an instance for the same (rule, exchange) still collapse
+-- to one — the repo's DuplicateKeyException fallback preserves defense-in-depth.
+DROP INDEX IF EXISTS alert_instances_open_rule_uq;
+
+CREATE UNIQUE INDEX alert_instances_open_rule_uq
+  ON alert_instances (rule_id, (COALESCE(context->'exchange'->>'id', '')))
+  WHERE rule_id IS NOT NULL
+    AND state IN ('PENDING','FIRING','ACKNOWLEDGED');