Optimize websocket broadcast fanout

[naufalfx805-source]

References #2

Scope

Optimizes the websocket fanout path in hub/ws/manager.py for both agent-specific broadcasts and price broadcasts.

Previously each subscriber was awaited one at a time, so one slow websocket delayed every other subscriber. The new path snapshots subscribers under the existing lock, sends the same payload concurrently with asyncio.gather(..., return_exceptions=True), and still removes sockets whose send fails.

I also replaced the runtime FastAPI WebSocket import with a small Protocol so the manager can be unit-tested without requiring FastAPI just for type annotations.

Benchmark

Synthetic local benchmark with 8 websocket subscribers, each send_text delayed by 30ms:

• Serial baseline: 0.261s
• Concurrent broadcast: 0.032s
• Improvement: 8.1x faster

This reflects the hot path where dashboard/price-feed clients are connected at the same time and one slow socket should not serialize the whole fanout.

Validation

• python -m pytest tests/test_ws_manager.py -q -> 2 passed
• git diff --cached --check -> passed

If selected for payout, I can provide a Base USDC address privately after acceptance.

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