Fix shutdown hang and blank equations when KaTeX is unavailable

Shutdown hang, the cause of every restart taking systemd's full 90 s timeout
and ending in SIGKILL: /api/stream looped forever with no disconnect or
shutdown check, so an open dashboard was an in-flight request that never
completed and uvicorn's graceful shutdown waited on it. Reproduced cleanly:
with no stream client the process stopped in 2 s, with one open client it was
still running after 15 s. Fixed by bounding timeout_graceful_shutdown, and by
having the generator exit on client disconnect and on a shutdown event. Now
7 s with a client attached.

Equations rendered as blank boxes whenever the KaTeX CDN was unreachable,
which is a real case for a Pi on wifi. The elements carried the TeX only in a
data attribute, so with no KaTeX there was nothing to display. The source is
now written into the element as text first and KaTeX replaces it, so it
degrades to readable TeX. Verified by aborting the katex request. A window
load handler re-runs typesetting for a slow CDN. The code comment claiming
this fallback already existed was wrong and is corrected.

ashvale/api.py

@@ -28,7 +28,7 @@ from contextlib import asynccontextmanager
 from typing import Any, Dict, List, Optional
 
 import numpy as np
-from fastapi import FastAPI, HTTPException, Query
+from fastapi import FastAPI, HTTPException, Query, Request
 from fastapi.responses import HTMLResponse, StreamingResponse
 from pydantic import BaseModel, Field
 
@@ -43,6 +43,14 @@ station: Optional[Station] = None
 display: Optional[LedDisplay] = None
 
 
+# Set when the app is shutting down. The SSE generator watches it: without
+# that, an open dashboard is an in-flight request that never completes, so
+# uvicorn's graceful shutdown blocks until systemd's timeout SIGKILLs the
+# process. Reproduced: with no stream client the service stops in 2 s, with
+# one open client it was still running after 15 s.
+_shutdown = asyncio.Event()
+
+
 @asynccontextmanager
 async def lifespan(app: FastAPI):
     global station, display
@@ -55,6 +63,7 @@ async def lifespan(app: FastAPI):
     try:
         yield
     finally:
+        _shutdown.set()
         if display is not None:
             await display.stop()
         if station is not None:
@@ -522,11 +531,18 @@ def events(limit: int = Query(50, ge=1, le=500)) -> List[Dict]:
 
 
 @app.get("/api/stream")
-async def stream():
+async def stream(request: Request):
     """Server-sent events. One connection instead of a poll every 2 seconds,
-    which on a Zero 2 W is the difference between 4% and 0.4% CPU."""
+    which on a Zero 2 W is the difference between 4% and 0.4% CPU.
+
+    The loop exits on shutdown or client disconnect. Both matter: an endless
+    generator keeps the response in flight, and uvicorn will not finish a
+    graceful shutdown while one is open.
+    """
     async def gen():
-        while True:
+        while not _shutdown.is_set():
+            if await request.is_disconnected():
+                break
             st = _st()
             payload = {
                 "telemetry": telemetry(),
@@ -535,7 +551,12 @@ async def stream():
                 "drift_stress": round(st.monitor.drift.stress, 3),
             }
             yield f"data: {json.dumps(payload)}\n\n"
-            await asyncio.sleep(2.0)
+            # Wait on the shutdown event rather than sleeping blindly, so a stop
+            # is honoured immediately instead of up to 2 s later.
+            try:
+                await asyncio.wait_for(_shutdown.wait(), timeout=2.0)
+            except asyncio.TimeoutError:
+                pass
 
     return StreamingResponse(gen(), media_type="text/event-stream",
                              headers={"Cache-Control": "no-cache",