feat: database layer + canvas validator/converter + LLM inference activities

- Add pkg/db models and CRUD methods for workflows
- Add internal/routing canvas validator (DAG check, connectivity)
- Add internal/routing canvas converter (Canvas → WorkflowSpec)
- Register LLMInferenceActivity and LLMBatchInferenceActivity
- Update api/server and cmd/server with database integration
- Add K8s environment variable support
- Update activity knowledge base with LLM activities
- Add .env.example configuration template

k8s/worker-deployment.yaml

@@ -13,8 +13,8 @@ spec:
       labels:
         app: poimen-worker
       annotations:
-        git-commit: "cdb6efe2"  # ✅ Updated on each push, triggers rolling restart
-        deployment-date: "2026-09-04"
+        git-commit: "e07b9504"  # ✅ Updated on each push, triggers rolling restart
+        deployment-date: "2026-09-05"
     spec:
       containers:
       - name: worker