feat: remove legacy alert rules, superseded by automations

Alert rules already auto-migrate into automations on boot and the old
engine stands down after. Deletes the rule/firing tables, routes, UI,
and AI tools; keeps notification channels since automations still use
them. Also fixes automations' notify step, which still called the
deleted repository.

src/backend/database/db/performance-indexes.ts

@@ -207,18 +207,6 @@ export const PERFORMANCE_INDEXES: PerformanceIndex[] = [
     columns: "user_id",
   },
 
-  // Alerting.
-  {
-    name: "idx_alert_firings_rule",
-    table: "alert_firings",
-    columns: "rule_id, fired_at",
-  },
-  {
-    name: "idx_alert_firings_host",
-    table: "alert_firings",
-    columns: "host_id",
-  },
-
   // Automations.
   {
     name: "idx_automations_user",