feat(backend): implement horizon failover, transaction index, auth rate limiting, and soroban event streamer (#1073)

- #947: StellarClient keeps a sticky primary plus backup RPC endpoints
  (STELLAR_RPC_BACKUP_URLS). Timeouts, connect errors, 5xx and 429 put the
  endpoint on a 30s cooldown and fail over immediately to the next healthy
  one; backoff applies only after a full pass over the pool.
- #948: Concurrent (zero-downtime) compound indexes on
  yield_transactions(user_id, created_at DESC) and
  payments(sender_id|receiver_id, created_at DESC), one per no-transaction
  migration, plus rollback scripts. The schema has no `transactions` table
  or `user_address` column, and the already-applied 20260728000000
  migration is left untouched to avoid a sqlx checksum mismatch.
- #949: Auth rate limiter is now a sliding window (10 req/min/IP) backed by
  an atomic Redis sorted-set Lua script, falling back to an in-process
  sliding window when Redis is unset or unreachable. 429 responses carry
  Retry-After.
- #950: indexer::soroban polls getEvents (via the failover client),
  decodes events with the indexer parser, and fans them out over a
  tokio broadcast channel to WebSocket clients at /api/events/ws. Lagging
  consumers skip ahead instead of blocking the producer. Enables axum "ws".

Co-authored-by: maryam <ipcymuhammad1@gmail.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: 3 people

backend/Cargo.toml

@@ -6,7 +6,7 @@ edition = "2021"
 [workspace]
 
 [dependencies]
-axum = { version = "0.7.4", features = ["multipart"] }
+axum = { version = "0.7.4", features = ["multipart", "ws"] }
 tokio = { version = "1.36.0", features = ["full"] }
 serde = { version = "1.0.197", features = ["derive"] }
 serde_json = "1.0.114"

backend/migrations/20260924000000_add_yield_transactions_user_created_at_index.sql

@@ -0,0 +1,14 @@
+-- no-transaction
+-- #948: Zero-downtime compound index for per-user transaction history
+-- (`WHERE user_id = $1 ORDER BY created_at DESC`).
+--
+-- CREATE INDEX CONCURRENTLY cannot run inside a transaction block, so the
+-- `-- no-transaction` directive above (must be the first line; sqlx 0.7
+-- syntax) tells SQLx not to wrap this file. Postgres also runs a
+-- multi-statement query string as one implicit transaction, so each
+-- concurrent index lives in its own migration file.
+--
+-- If a concurrent build is interrupted it leaves an INVALID index that
+-- IF NOT EXISTS will skip; drop it with DROP INDEX CONCURRENTLY and re-run.
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_yield_tx_user_id_created_at
+    ON yield_transactions (user_id, created_at DESC);

backend/migrations/20260924000001_add_payments_sender_created_at_index.sql

@@ -0,0 +1,7 @@
+-- no-transaction
+-- #948: Zero-downtime compound index for a user's sent-payment history
+-- (`WHERE sender_id = $1 ORDER BY created_at DESC`). See
+-- 20260924000000_add_yield_transactions_user_created_at_index.sql for why
+-- this runs outside a transaction and in its own file.
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_payments_sender_id_created_at
+    ON payments (sender_id, created_at DESC);

backend/migrations/20260924000002_add_payments_receiver_created_at_index.sql

@@ -0,0 +1,7 @@
+-- no-transaction
+-- #948: Zero-downtime compound index for a user's received-payment history
+-- (`WHERE receiver_id = $1 ORDER BY created_at DESC`). See
+-- 20260924000000_add_yield_transactions_user_created_at_index.sql for why
+-- this runs outside a transaction and in its own file.
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_payments_receiver_id_created_at
+    ON payments (receiver_id, created_at DESC);

backend/migrations/rollback/20260924000000_add_yield_transactions_user_created_at_index.down.sql

@@ -0,0 +1,3 @@
+-- Rollback for 20260924000000_add_yield_transactions_user_created_at_index.sql
+-- Run outside a transaction (psql -f does not wrap by default).
+DROP INDEX CONCURRENTLY IF EXISTS idx_yield_tx_user_id_created_at;

backend/migrations/rollback/20260924000001_add_payments_sender_created_at_index.down.sql

@@ -0,0 +1,3 @@
+-- Rollback for 20260924000001_add_payments_sender_created_at_index.sql
+-- Run outside a transaction (psql -f does not wrap by default).
+DROP INDEX CONCURRENTLY IF EXISTS idx_payments_sender_id_created_at;

backend/migrations/rollback/20260924000002_add_payments_receiver_created_at_index.down.sql

@@ -0,0 +1,3 @@
+-- Rollback for 20260924000002_add_payments_receiver_created_at_index.sql
+-- Run outside a transaction (psql -f does not wrap by default).
+DROP INDEX CONCURRENTLY IF EXISTS idx_payments_receiver_id_created_at;