[ci skip] f1-stream: add stream health checker and HLS proxy (Phases 4-5)

Phase 4 - Stream Health and Fallback:
- StreamHealthChecker with partial GET validation of m3u8 content
- Bitrate extraction from BANDWIDTH tags
- Response time measurement for quality ranking
- Fallback ordering: live first, fastest response time first
- GET /streams now only returns health-verified streams

Phase 5 - HLS Proxy Core:
- GET /proxy?url= - m3u8 playlist fetch with full URI rewriting
- GET /relay?url= - chunked segment relay (never buffers full segment)
- m3u8 rewriter handles master, variant, and segment URIs
- Base64url encoding for URL parameters
- CORS middleware for browser playback
- Range header forwarding for seeking support

stacks/f1-stream/files/backend/extractors/models.py

@@ -15,6 +15,9 @@ class ExtractedStream:
     title: str = ""  # e.g., "F1 Race Live"
     extracted_at: str = field(default_factory=lambda: datetime.now(timezone.utc).isoformat())
     is_live: bool = False  # Whether it passed health check
+    response_time_ms: int = 0  # Health check response time (lower = better)
+    checked_at: str = ""  # ISO timestamp of last health check
+    bitrate: int = 0  # Bitrate in bps if detectable from m3u8 playlist
 
     def to_dict(self) -> dict:
         """Serialize to a plain dictionary for JSON responses."""
@@ -26,4 +29,7 @@ class ExtractedStream:
             "title": self.title,
             "extracted_at": self.extracted_at,
             "is_live": self.is_live,
+            "response_time_ms": self.response_time_ms,
+            "checked_at": self.checked_at,
+            "bitrate": self.bitrate,
         }