[ci skip] f1-stream: add F1 schedule subsystem (Phase 2)

- Fetch 2026 F1 race calendar from jolpica API with all sessions
  (FP1-3, Qualifying, Sprint, Race) and UTC timestamps
- Persist schedule to NFS as JSON, load on startup if fresh
- APScheduler daily refresh at 03:00 UTC
- GET /schedule endpoint with live/upcoming/past session status
- POST /schedule/refresh for manual refresh trigger

stacks/f1-stream/files/backend/requirements.txt

@@ -1,2 +1,4 @@
 fastapi==0.115.0
 uvicorn[standard]
+httpx>=0.27.0
+apscheduler>=3.10.0,<4.0