Concepts docs: Gateway architecture — rooms, resume, rate limiting

[codebestia]

Write apps/backend/docs/concepts-gateway-architecture.md explaining the WebSocket gateway's structural pieces: services/roomManager.ts (conversation + user rooms), services/resumeStream.ts (Redis Streams-backed missed-event replay), services/rateLimit.ts (per-socket rate limiting/payload caps), services/backpressure.ts (slow-consumer handling), and the Redis pub/sub cross-node fan-out.

Acceptance criteria:

• Room types and their purpose are explained, including why membership is re-validated against Postgres rather than trusted from room state alone
• The resume/replay mechanism is explained with its bounding/TTL behavior
• Rate limiting, payload caps, and backpressure/shedding thresholds are documented with their current configurable values and env var names

[jaymoneymanxl]

@jaymoneymanxl has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

I would love to work on this, Please assign it to me

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[drips-wave]

Congratulations, @jaymoneymanxl! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

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