docs(alerting): PER_EXCHANGE exactly-once — fireMode reference + deploy-backlog-cap

Fix stale `AGGREGATE` label (actual enum: `COUNT_IN_WINDOW`). Expand
EXCHANGE_MATCH section with both fire modes, PER_EXCHANGE config-surface
restrictions (0 for reNotifyMinutes/forDurationSeconds, at-least-one-sink
rule), exactly-once guarantee scope, and the first-run backlog-cap knob.

Surface the new config in application.yml with the 24h default and the
opt-out-to-0 semantics.

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

cameleer-server-app/src/main/resources/application.yml

@@ -93,6 +93,10 @@ cameleer:
       notification-retention-days: ${CAMELEER_SERVER_ALERTING_NOTIFICATIONRETENTIONDAYS:30}
       webhook-timeout-ms: ${CAMELEER_SERVER_ALERTING_WEBHOOKTIMEOUTMS:5000}
       webhook-max-attempts: ${CAMELEER_SERVER_ALERTING_WEBHOOKMAXATTEMPTS:3}
+      # PER_EXCHANGE first-run cursor clamp: on first tick with no persisted cursor, evaluator
+      # scans no further back than (now - this cap). Prevents one-time backlog flood for rules
+      # whose createdAt predates a migration. Set to 0 to disable and replay from createdAt.
+      per-exchange-deploy-backlog-cap-seconds: ${CAMELEER_SERVER_ALERTING_PEREXCHANGEDEPLOYBACKLOGCAPSECONDS:86400}
     outbound-http:
       trust-all: false
       trusted-ca-pem-paths: []