Add dd0c/drift notifications, infra, CI: Slack Block Kit, Dockerfiles, Gitea Actions

- Notification service: Slack Block Kit (remediate/accept buttons), webhook delivery, rate limit handling
- Dispatcher with severity-based channel filtering
- Agent Dockerfile: multi-stage Go build, static binary
- SaaS Dockerfile: multi-stage Node build
- Fly.io config: scale-to-zero, shared-cpu
- Gitea Actions: Go test+vet, Node typecheck+test, cross-compile agent (linux/darwin/windows)

products/02-iac-drift-detection/saas/fly.toml

@@ -0,0 +1,27 @@
+app = "dd0c-drift"
+primary_region = "iad"
+
+[build]
+  dockerfile = "Dockerfile"
+
+[env]
+  NODE_ENV = "production"
+  PORT = "3000"
+  LOG_LEVEL = "info"
+
+[http_service]
+  internal_port = 3000
+  force_https = true
+  auto_stop_machines = true
+  auto_start_machines = true
+  min_machines_running = 0
+
+  [http_service.concurrency]
+    type = "requests"
+    hard_limit = 100
+    soft_limit = 80
+
+[[vm]]
+  cpu_kind = "shared"
+  cpus = 1
+  memory_mb = 256