Rotate away from a connected server that stopped following the chain (B-86, ELC-21, ARC-26)

`server.ping` keeps succeeding on a server that is stalled, and nothing compared its tip
with the others', so it stayed the active server for ever.

- The listener records when the last header was accepted (monotonic clock, a module
  attribute so tests can move it).
- After every successful ping, ElectrumListener._check_not_stalled: once the last accepted
  header is older than 360s the other servers are asked for their tip
  (blockchain.headers.subscribe, ELC-14's timeouts); if any reports a higher height than
  ours the session ends with ServerStalledError and run() rotates to the next server. If
  none does the chain is just quiet and the session goes on.

No existing test changed. ELC-29 and /admin/electrum (the connection state and the age of
the last header, B-92) are untouched.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

BUGS.md

@@ -166,15 +166,6 @@ winner's own bet is gone the payout is held (`payout_blocked`) and the lottery p
 
 ## High
 
-### B-86 — A connected but stalled server is never replaced
-
-`ping` keeps succeeding while the server stops following the chain; nothing compares
-tip progress with the other servers, and a restart goes back to the primary.
-
-Fix: if the tip has not advanced for 360s and any other server reports a higher tip,
-rotate. `GET /health/ready` and `/admin` expose connection state, endpoint and
-last-header age (B-92).
-
 ### B-87 — The RBF bump can leave a dust change output
 
 `bump_fee` only requires change > fee delta. A 294-sat change (full-balance