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)

backend/app/main.py

@@ -1,5 +1,6 @@
 from fastapi import FastAPI
 
+from .flows.api import approvals_router, flows_router
 from .settings import settings
 
 app = FastAPI(
@@ -11,6 +12,9 @@ app = FastAPI(
     openapi_url="/api/openapi.json",
 )
 
+app.include_router(flows_router)
+app.include_router(approvals_router)
+
 
 @app.get("/")
 async def root():