feat(server): session-sync relay WebSocket /ws/sync/{session_id} (#1030) (#1032)

* feat(server): add session-sync relay WebSocket /ws/sync/{session_id}

Cross-device followers (splitscreen's upcoming LAN pop-out mode,
feedBack-plugin-splitscreen#21) need a machine-crossing replacement for
BroadcastChannel — the one link in the follower architecture that cannot
leave the host browser. Chart data already streams per-client over
/ws/highway, so all that's missing is a dumb live-state channel.

Add a fan-out room endpoint: a JSON text frame from one client is relayed
verbatim to every other client on the same session id. No schema, no
history, no persistence — rooms are created on first join and GC'd when
the last socket leaves. The statelessness is deliberate: an idle room is
indistinguishable from a nonexistent one, and a host that crashes and
rejoins the same id resumes publishing to reconnecting subscribers with
no server-side coordination.

Caps for a LAN-exposable port: 16 KB frames (1009), 16 sockets/room and
32 rooms (1013), 120 msg/s sustained / 240 burst per socket (1008),
text-only (1003), session id validated against [A-Za-z0-9_-]{4,64}. An
over-limit socket is closed individually; a peer that dies mid-fan-out
is dropped without wedging delivery to the rest.

Closes #1030

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Kris Anderson <topkoa@gmail.com>

* fix(ws_sync): bound stalled peer sends; cap ws frames at the transport

Review feedback (CodeRabbit on #1032):

- A peer that stops draining its socket left send_text() pending forever;
  since publishers await the fan-out gather, one stalled peer stalled
  every publisher's receive loop behind it. Fan-out sends are now bounded
  by SEND_TIMEOUT_SECONDS (5 s) so a stall becomes an eviction through
  the existing failed-send drop path.

- uvicorn buffers inbound WS frames up to its 16 MB default before the
  handler's 16 KB check ever runs, so the DoS bound wasn't enforced at
  the transport. main.py now passes ws_max_size=64 KB (no client sends
  large frames: the highway WS receives only small control messages, and
  the relay keeps its tighter application cap as the primary limit).

Regression tests for both; the desktop's own uvicorn spawn gets the
matching --ws-max-size flag with the feedBack-desktop follow-up work.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Kris Anderson <topkoa@gmail.com>

---------

Signed-off-by: Kris Anderson <topkoa@gmail.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -8,6 +8,24 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- **Session-sync relay WebSocket — `/ws/sync/{session_id}` (#1030).** A
+  deliberately dumb JSON fan-out room: a text frame received from one client is
+  forwarded verbatim to every other client on the same session id; the server
+  interprets nothing (message schemas are owned by consumers). Rooms are created
+  on first join and garbage-collected when the last socket leaves — no history,
+  no replay, no persistence, so a host that crashes and rejoins the same id
+  resumes publishing to reconnecting subscribers with no server-side
+  coordination. Session ids are client-generated (`[A-Za-z0-9_-]{4,64}`); DoS
+  hygiene for a LAN-exposed port via frame-size (16 KB), per-room (16 sockets),
+  total-room (32), and per-socket rate (120 msg/s sustained, 240 burst) caps —
+  over-limit sockets are closed with a policy code and the room carries on, and
+  a peer that dies — or stalls: fan-out sends are bounded by a 5 s timeout —
+  mid-fan-out is dropped without disturbing delivery to the rest. `main.py`
+  also caps inbound WS frames at the transport (`ws_max_size=64 KB`, down from
+  uvicorn's 16 MB default) so oversized frames never materialize server-side. First consumer: splitscreen's "pop out to LAN" follower mode
+  (feedBack-plugin-splitscreen#21), which relays playhead/playstate/song-change
+  frames from a host window to view-only followers on other LAN devices.
+  Implementation in `lib/routers/ws_sync.py`; tests in `tests/test_ws_sync.py`.
 - **Drum-part picker (feedpak 1.17.0 "drums as arrangements").** When a song
   carries several drum charts, a **Drum part** selector appears beside the
   arrangement switcher (advanced settings) so a player can choose which drummer