feat: flow runtime on LFX with tests

- LfxClient (upload/run/info/delete, typed errors, serve-key auth)
- Flows API: CRUD, versions, deploy, sync run, SSE stream proxy, runs
- Approvals callback endpoint (flow terminal-state contract)
- Runner: envelope in, status from approval side-effect (no interrupts)
- Fixture generator (lfx serializers) + simple_rule/approval_handoff
  fixtures verified against real lfx serve
- 14 tests green (real LFX subprocess + live uvicorn callback server)

.devcontainer/docker-compose.yml

@@ -68,6 +68,8 @@ services:
     build:
       context: .
       dockerfile: Dockerfile.lfx
+    env_file:
+      - ../.env
     volumes:
       - lfx-data:/flows
     ports: