[ibrahimMaryam-tech]

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## Summary of Changes

1. **Stellar Horizon Failover Manager (#947):**
   - Updated `backend/src/services/stellar.rs` with round-robin failover pool across multiple Horizon RPC endpoints upon timeouts or 5xx server errors.

2. **Transaction Database Index Migration (#948):**
   - Added migration `backend/migrations/20260728000000_add_users_created_at_index.sql` adding `CREATE INDEX CONCURRENTLY` on `transactions (user_address, created_at DESC)`.

3. **Auth Rate Limiting Middleware (#949):**
   - Implemented Redis sliding window rate limiter in `backend/src/api/auth.rs` restricting IP attempts to 10 requests/minute, returning HTTP 429 on overflow.

4. **Soroban Event Streamer (#950):**
   - Built Soroban event subscriber in `backend/src/indexer/soroban.rs` consuming `getEvents` and broadcasting decoded events over a `tokio::sync::broadcast` channel to WebSocket frontend clients.

## Issue Resolution

Closes #947
Closes #948
Closes #949
Closes #950

## Verification

- [x] Horizon failover logic implemented in `stellar.rs`
- [x] Non-blocking PostgreSQL migration created
- [x] Redis sliding window rate limiter implemented in Axum middleware
- [x] Tokios broadcast channel event streamer added for Soroban ledger events

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Great job @ibrahimMaryam-tech