feat: remove legacy alert rules, superseded by automations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.

drizzle/postgres/0026_perfect_masque.sql

@@ -0,0 +1,3 @@
+DROP TABLE "alert_firings" CASCADE;--> statement-breakpoint
+DROP TABLE "alert_rule_channels" CASCADE;--> statement-breakpoint
+DROP TABLE "alert_rules" CASCADE;