fix(alerting): reject null fireMode on ExchangeMatchCondition + repair in-flight rows

The rule editor wizard reset the condition payload on kind-change without
seeding a fireMode default; the ExchangeMatchCondition ctor allowed null to
pass through; AlertEvaluatorJob then NPE-looped every tick on a saved rule.

- core: compact ctor now rejects null fireMode (Jackson-deser path only — all
  production callers already pass a value).
- V14: repair existing EXCHANGE_MATCH rows with fireMode=null to
  PER_EXCHANGE + perExchangeLingerSeconds=300 (default matches the wizard).
- ui: ConditionStep.onKindChange seeds EXCHANGE_MATCH defaults so the
  Select's displayed fallback ("Per exchange") is actually in form state.
- ui: validateStep('condition', ...) now enforces fireMode presence + the
  mode-specific fields before the user reaches Review.

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

CLAUDE.md

@@ -70,6 +70,7 @@ PostgreSQL (Flyway): `cameleer-server-app/src/main/resources/db/migration/`
 - V11 — Outbound connections (outbound_connections table, enums)
 - V12 — Alerting tables (alert_rules, alert_rule_targets, alert_instances, alert_notifications, alert_reads, alert_silences)
 - V13 — alert_instances open-rule unique index (alert_instances_open_rule_uq partial index on rule_id WHERE state IN PENDING/FIRING/ACKNOWLEDGED)
+- V14 — Repair EXCHANGE_MATCH alert_rules persisted with fireMode=null (sets fireMode=PER_EXCHANGE + perExchangeLingerSeconds=300); paired with stricter `ExchangeMatchCondition` ctor that now rejects null fireMode.
 
 ClickHouse: `cameleer-server-app/src/main/resources/clickhouse/init.sql` (run idempotently on startup)