Proxy: log slow segment transfers (TTFB>3s or total>8s)

Distinguishes CDN-side stalls (server sees slow TTFB) from client-path
stalls (server fast, user still times out) when fragLoadTimeOut recurs.

server/app.py

@@ -488,9 +488,12 @@ def _cache_put(url, ct, body):
 def _seg_stream(r, url, ct):
     """Forward the CDN's bytes as they arrive; keep the LRU warm for
     completed downloads (a truncated one would poison the cache)."""
-    buf, complete = [], False
+    import time
+    t0, t_first, buf, complete = time.time(), None, [], False
     try:
         for c in r.iter_content(chunk_size=256 * 1024):
+            if t_first is None:
+                t_first = time.time()
             buf.append(c)
             yield c
         complete = True
@@ -498,6 +501,17 @@ def _seg_stream(r, url, ct):
         r.close()
         if complete and buf:
             _cache_put(url, ct, b"".join(buf))
+        # observability: a slow segment is the fragLoadTimeOut culprit. Log
+        # only the slow ones so normal playback stays quiet. TTFB>3s or
+        # total>8s means the CDN (or our upstream hop) stalled — that's what
+        # the browser times out on. If these show ~0s but the user still
+        # fails, the stall is in *their* path (HTTPS proxy / network), not ours.
+        ttfb = (t_first - t0) if t_first is not None else None
+        total = time.time() - t0
+        if (ttfb is not None and ttfb > 3) or total > 8:
+            name = url.rsplit("/", 1)[-1][:40]
+            print(f"[stream-slow] {name} ttfb={ttfb:.1f}s total={total:.1f}s "
+                  f"bytes={sum(len(b) for b in buf)} complete={complete}", flush=True)
 
 
 def _stream_playlist(url):