Stop open SSE streams from blocking graceful shutdown

Every connected dashboard holds a StreamingResponse open indefinitely, so uvicorn's
graceful shutdown waited on them, systemd hit its 90s stop timeout and SIGKILLed the
unit. That skipped the in-process restore hook entirely, leaving the GPU restore to
ExecStopPost alone.

- broker.stop() sets a closing flag and pushes a sentinel to every subscriber queue so
  the SSE generators return instead of parking on q.get().
- uvicorn gets timeout_graceful_shutdown=10 and the unit TimeoutStopSec=20, bounding
  the worst case rather than relying on the 90s default.

Restart now completes in ~11s with 'Restoring GPU to safe stock state (server
shutdown)' running in-process, and no SIGKILL.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

server.py

@@ -55,6 +55,8 @@ class TelemetryBroker:
         self.running = False
         self.samples = 0
         self.last_sample_ms = 0.0
+        # Set on shutdown so open SSE generators finish instead of holding the server up.
+        self.closing = False
 
     async def start(self) -> None:
         if self.running:
@@ -64,6 +66,13 @@ class TelemetryBroker:
 
     async def stop(self) -> None:
         self.running = False
+        self.closing = True
+        # Wake every subscriber so their generator can return. Without this, uvicorn waits
+        # on the open SSE responses during graceful shutdown and systemd eventually
+        # SIGKILLs the unit -- which skips the in-process GPU restore hook entirely.
+        for q in list(self.subscribers):
+            with contextlib.suppress(asyncio.QueueFull):
+                q.put_nowait(None)
         if self.task:
             self.task.cancel()
             with contextlib.suppress(asyncio.CancelledError):
@@ -250,11 +259,13 @@ async def sse_telemetry_stream(request: Request):
         try:
             snap = await broker.get()
             yield f"data: {json.dumps(snap)}\n\n"
-            while True:
+            while not broker.closing:
                 if await request.is_disconnected():
                     break
                 try:
                     snap = await asyncio.wait_for(q.get(), timeout=15.0)
+                    if snap is None:      # shutdown sentinel
+                        break
                     yield f"data: {json.dumps(snap)}\n\n"
                 except asyncio.TimeoutError:
                     yield ": keepalive\n\n"
@@ -497,4 +508,5 @@ async def root_index():
 
 if __name__ == "__main__":
     import uvicorn
-    uvicorn.run("server:app", host="0.0.0.0", port=9090, reload=False, log_level="info")
+    uvicorn.run("server:app", host="0.0.0.0", port=9090, reload=False, log_level="info",
+                timeout_graceful_shutdown=10)