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

.env.example

@@ -0,0 +1,22 @@
+# Workflows Backend Configuration
+
+# Database (memory-db CNPG in K8s)
+# Option A: Direct DATABASE_URL
+DATABASE_URL=postgresql://app:[email protected]:5432/memory?sslmode=disable
+
+# Option B: Individual env vars (used if DATABASE_URL is empty)
+DATABASE_HOST=memory-db-rw.poimen.svc.cluster.local
+DATABASE_PORT=5432
+DATABASE_NAME=memory
+DATABASE_USER=app
+DATABASE_PASSWORD=PASSWORD
+
+# Temporal
+TEMPORAL_HOST_PORT=localhost:7233
+TEMPORAL_NAMESPACE=default
+
+# API Server
+API_PORT=8080
+
+# Logging
+VERBOSE=false