docs(backend): refresh SSE endpoint guidance

backend/SSE_README.md

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+# Server-Sent Events (SSE)
+
+FlowFi uses Server-Sent Events for real-time payment-stream updates. SSE is a
+good fit for these server-to-client notifications because it uses standard
+HTTP, reconnects automatically, and has less overhead than a bidirectional
+WebSocket connection.
+
+## Implementation
+
+The SSE implementation includes:
+
+- subscription filtering by stream, user, or all of the authenticated user's streams;
+- automatic connection cleanup and heartbeats;
+- per-IP and global connection limits;
+- slow-client protection;
+- Redis pub/sub for multi-instance delivery, with an in-memory fallback; and
+- connection statistics for monitoring.
+
+The relevant implementation lives in:
+
+- [`src/routes/v1/events.routes.ts`](src/routes/v1/events.routes.ts) — authenticated v1 event routes;
+- [`src/controllers/sse.controller.ts`](src/controllers/sse.controller.ts) — subscription validation, ownership filtering, and capacity enforcement;
+- [`src/services/sse.service.ts`](src/services/sse.service.ts) — connection lifecycle and event delivery;
+- [`src/lib/redis.ts`](src/lib/redis.ts) — Redis publisher/subscriber clients; and
+- [`src/index.ts`](src/index.ts) — Redis startup and graceful shutdown.
+
+## Authentication and versioning
+
+`GET /v1/events/subscribe` requires a valid JWT in the
+`Authorization: Bearer <token>` header. Subscriptions are restricted to streams
+owned by the authenticated wallet.
+
+> The old unversioned `GET /events/subscribe` path is deprecated and returns
+> HTTP `410 Gone`. Use `/v1/events/subscribe`; it requires JWT authentication.
+
+All state-changing stream endpoints are likewise authenticated and versioned
+under `/v1/streams`.
+
+## API endpoints
+
+### Subscribe to events
+
+```http
+GET /v1/events/subscribe?streams=1&streams=2
+GET /v1/events/subscribe?users=GABC...
+GET /v1/events/subscribe?all=true
+Authorization: Bearer <JWT>
+```
+
+The `all=true` option means all streams visible to the authenticated wallet,
+not every stream in the system.
+
+### Connection statistics
+
+```http
+GET /v1/events/stats
+```
+
+### Authenticated stream operations
+
+```http
+POST /v1/streams
+POST /v1/streams/:streamId/top-up
+POST /v1/streams/:streamId/pause
+POST /v1/streams/:streamId/resume
+POST /v1/streams/:streamId/withdraw
+POST /v1/streams/:streamId/cancel
+Authorization: Bearer <JWT>
+```
+
+## Event types
+
+- `stream.created`
+- `stream.topped_up`
+- `stream.withdrawn`
+- `stream.cancelled`
+- `stream.completed`
+- `stream.paused`
+- `stream.resumed`
+
+## Quick test
+
+Start the backend:
+
+```bash
+cd backend
+npm run dev
+```
+
+Set a JWT obtained from the v1 authentication flow, then subscribe:
+
+```bash
+export TOKEN="<JWT>"
+
+curl -N \
+  -H "Authorization: Bearer ${TOKEN}" \
+  "http://localhost:3001/v1/events/subscribe?all=true"
+```
+
+In another terminal, create a stream through the authenticated v1 endpoint.
+The `sender` must match the wallet represented by the JWT:
+
+```bash
+curl -X POST "http://localhost:3001/v1/streams" \
+  -H "Authorization: Bearer ${TOKEN}" \
+  -H "Content-Type: application/json" \
+  -d '{
+    "streamId": "example-stream-1",
+    "sender": "GABC...",
+    "recipient": "GDEF...",
+    "tokenAddress": "CUSDC...",
+    "ratePerSecond": "1000000",
+    "depositedAmount": "86400000000",
+    "startTime": "1708560000"
+  }'
+```
+
+## Production considerations
+
+### Security
+
+- [x] JWT authentication on `/v1/events/subscribe` — [`src/routes/v1/events.routes.ts`](src/routes/v1/events.routes.ts)
+- [x] Per-IP connection limits — [`src/controllers/sse.controller.ts`](src/controllers/sse.controller.ts) and [`src/services/sse.service.ts`](src/services/sse.service.ts)
+- [ ] Use a reverse proxy (nginx or Cloudflare) for additional DDoS protection
+
+### Scaling
+
+- [x] Redis pub/sub for multi-instance deployments — [`src/lib/redis.ts`](src/lib/redis.ts) and [`src/services/sse.service.ts`](src/services/sse.service.ts)
+- [ ] Monitor connection count and configure alerts
+- [ ] Tune connection limits from production traffic data
+
+### Monitoring
+
+- [ ] Track active connections in the production monitoring system
+- [ ] Monitor events per second
+- [ ] Alert on reconnection spikes
+
+## Further documentation
+
+See [`docs/SSE_IMPLEMENTATION.md`](docs/SSE_IMPLEMENTATION.md) for client
+integration, reconnection, scaling, and operational guidance.