feat(go-api): WebSocket endpoint with Redis Streams fan-out

[Depo-dev]

Summary

Implements GET /ws WebSocket endpoint with per-connection Redis Streams fan-out.

Library: github.com/gorilla/websocket — chosen over golang.org/x/net/websocket for built-in ping/pong control frame support, production hardening, and a stable documented API.

Architecture

Client → GET /ws?contractId=...&topic0=...
         ↓ Hub.Handler upgrades HTTP → WS
         ↓ newClient().run() spawns:
           - drain goroutine (reads WS to receive pong frames)
           - pingLoop (30s interval ping, 60s pong wait)
           - redisReadLoop (XREAD BLOCK 5s, filters by contractId + topic0, WriteJSON)
         ↓ On disconnect → cancel ctx → unregister from Hub → conn.Close()

Files

• ws/hub.go — Hub with RWMutex-protected client set, Register/Unregister, Handler
• ws/client.go — Client with Redis fan-out goroutine
• ws/hub_test.go — 4 tests (all pass, go vet clean)

Tests

Test	What it verifies
TestHub_missingContractID_returns400	Missing param → 400 before upgrade
TestHub_connectDisconnect_lifecycle	Register → count=1, unregister → count=0
TestHub_multipleClients	5 clients registered/unregistered correctly
TestHub_websocketConnect_receivesEvent	Upgrade path + server-side close propagates

Closes #15