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

[Depo-dev]

Summary

Real-time event delivery to SDK clients requires a WebSocket endpoint backed by a Redis Streams consumer.

Acceptance Criteria

• GET /ws — upgrades to WebSocket; accepts contractId and optional topic0 query params
• Background goroutine per connection: XREAD BLOCK on trident:events stream, filter by contractId/topic0, write JSON to the WebSocket
• Hub manages all active connections; cleans up on disconnect
• Ping/pong keepalive (60s timeout)
• Tests for the hub: connect, receive event, disconnect lifecycle

Files

services/api/ws/hub.go (new), services/api/ws/client.go (new)

Dependencies

Requires github.com/gorilla/websocket or stdlib golang.org/x/net/websocket (decide and document).