Fan out WebSocket broadcast sends to all peers concurrently

[groupthinking]

Make WebSocketConnectionManager.broadcast issue send_text calls in parallel so slow peers no longer cause head-of-line blocking.

[linear-code]

GRV-223

[coderabbitai]

⚠️ Possible Duplicate Issue(s)

• perf(websocket): broadcast() serialises sends, causing head-of-line blocking across all clients #1185

---

📝 Issue Planner

_{Check the box below or use the @coderabbitai plan command to generate an implementation plan and prompts that you can use with your favorite coding assistant.}

• Create Plan

---

🧪 Issue enrichment is currently in open beta.

You can configure auto-planning by selecting labels in the issue_enrichment configuration.

To disable automatic issue enrichment, add the following to your .coderabbit.yaml:

issue_enrichment:
  auto_enrich:
    enabled: false

💬 Have feedback or questions? Drop into our discord!

[groupthinking]

Already resolved on main — closing as complete

This was filed on 2026-08-01, the same day the fix merged. The tracking issue was never
linked to the PR that implemented it, so it stayed open. The work described here is
already shipped; leaving it open risks somebody re-implementing it.

Fix: 9e405a39b (#1186) — perf(websocket): broadcast to all peers concurrently instead of one at a time
Code: src/youtube_extension/backend/services/websocket_service.py

WebSocketConnectionManager.broadcast now snapshots the connection list and issues every send_text through a single asyncio.gather(..., return_exceptions=True), so a slow peer no longer head-of-line blocks the fleet. Dead peers are disconnected individually and CancelledError is re-raised rather than swallowed.

Verification performed

1. Commit is on main — git merge-base --is-ancestor 9e405a39b origin/main exits 0.

2. Mechanism present in current origin/main — confirmed by reading
   git show origin/main:src/youtube_extension/backend/services/websocket_service.py, not just the merge record.

3. Regression suite shipped with the fix passes:

   pytest tests/unit/test_websocket_service.py
   # 61 passed
   

No further work required.

_{Verified by an automated triage pass. The whole async-fix cluster
(#1191, #1192, #1198, #1199, #1201, #1206) was re-run together: 442 passed, 0 failed.}