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/Dockerfile

@@ -0,0 +1,14 @@
+FROM node:22-slim AS builder
+WORKDIR /app
+COPY package.json package-lock.json* ./
+RUN npm ci
+COPY . .
+RUN npm run build
+
+FROM node:22-slim
+WORKDIR /app
+COPY --from=builder /app/dist ./dist
+COPY --from=builder /app/node_modules ./node_modules
+COPY --from=builder /app/package.json ./
+EXPOSE 3000
+CMD ["node", "dist/index.js"]