Background CDN probe: log throughput + state flips every 3 min

64KB Range fetch from a fixed episode (JJK S2 E25) on a daemon thread;
logs '[probe] CDN OK/DEGRADED (N KB/s)' on state change so server.log
shows exactly when a degraded window ends. 18/18 tests, verified live.

server/app.py

@@ -7,6 +7,7 @@ Cloudflare workaround) and rewrites playlist URIs to go through itself.
 import base64
 import json
 import os
+import threading
 import time
 from collections import Counter, OrderedDict
 from datetime import datetime
@@ -24,6 +25,42 @@ from . import db, upstream, users
 ROOT = Path(__file__).parent.parent
 app = FastAPI(title="ani-web")
 
+
+def _cdn_probe():
+    """Background probe: pull 64KB from the CDN every 3 min and log state
+    flips, so server.log shows when a degraded window ends (CDN RECOVERED).
+    Probe target: any valid episode (36019 = JJK S2 E25).
+    ponytail: single fixed episode; if it ever disappears, the probe logs
+    DEGRADED forever — swap the ID, don't build a picker."""
+    def first(pl, base):
+        l = next(x.strip() for x in pl.splitlines()
+                 if x.strip() and not x.startswith("#"))
+        return l if l.startswith("http") else base + l
+
+    state = None
+    while True:
+        try:
+            m = upstream.master_m3u8(36019, "jpn")
+            if not m:
+                raise RuntimeError("no master m3u8")
+            base = m.rsplit("/", 1)[0] + "/"
+            v = first(upstream._get(m, timeout=10).text, base)
+            seg = first(upstream._get(v, timeout=10).text, base)
+            t = time.time()
+            r = upstream._session().get(seg, headers={"Range": "bytes=0-65535"},
+                                        impersonate="chrome", timeout=10)
+            kbps = len(r.content) / 1024 / max(time.time() - t, 0.01)
+        except Exception:
+            kbps = 0.0
+        now = "OK" if kbps >= 40 else "DEGRADED"
+        if now != state:
+            print(f"[probe] CDN {now} ({kbps:.0f} KB/s)", flush=True)
+        state = now
+        time.sleep(180)
+
+
+threading.Thread(target=_cdn_probe, daemon=True).start()
+
 # ---------- auth: multi-user, signed cookies ----------
 users.migrate_legacy()  # file-based legacy import; db.init() absorbs its output
 db.init()