feat: telemetry alert rules with webhook delivery

core.alert_rules (migration 00012) — heartbeat_silence and error_spike
rules evaluated by a 60s backend ticker. Transition-based firing:
alert.fired on breach, alert.resolved on recovery, both first-class
webhook event types riding the signed dispatcher + realtime stream.
Dashboard manages rules on the Integrations page.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/db/migrations/00012_alert_rules.sql

@@ -0,0 +1,16 @@
+-- +goose Up
+CREATE TABLE core.alert_rules (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  project_id UUID NOT NULL REFERENCES core.projects(id) ON DELETE CASCADE,
+  name TEXT NOT NULL,
+  kind TEXT NOT NULL CHECK (kind IN ('heartbeat_silence','error_spike')),
+  config JSONB NOT NULL DEFAULT '{}'::jsonb,
+  enabled BOOLEAN NOT NULL DEFAULT TRUE,
+  state JSONB NOT NULL DEFAULT '{}'::jsonb,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  UNIQUE (project_id, name)
+);
+
+-- +goose Down
+DROP TABLE IF EXISTS core.alert_rules;