Add Redis pub/sub for horizontal SSE scaling

[ogazboiz]

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Description

SSE broadcasts are in-process only. When the backend runs as multiple instances (e.g., behind a load balancer), a stream event processed by instance A is never broadcast to clients connected to instance B.

What Needs to Happen

• Install ioredis and configure Redis pub/sub
• On indexer event, publish to Redis channel stream:{streamId} and user:{address}
• Each instance subscribes to relevant channels and broadcasts to its local SSE clients
• Add REDIS_URL to .env.example with a graceful fallback to in-process mode when Redis is absent
• Document the multi-instance deployment pattern in README

Files

• backend/src/services/sseService.ts
• backend/src/lib/redis.ts (create)

Acceptance Criteria

• Events delivered to clients on any instance
• Works without Redis (single-instance fallback)
• Connection pool and retry configured

[JONAH-6]

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