feat: graceful shutdown, circuit breaker, retry backoff & batch tx support

[covenantalade]

Closes #54

Closes #55

Closes #56

Closes #57

What changed

#57 – Graceful shutdown

• SIGTERM/SIGINT handlers stop all cron tasks, drain in-flight HTTP
  requests via server.close(), then drain the RPC pool (10 s timeout)
• All cron schedules registered through a scheduleCron() wrapper so
  tasks are tracked and can be stopped cleanly
• isShuttingDown guard prevents crons from starting new work mid-shutdown
• Each shutdown step is logged for observability

#56 – Circuit breaker for Stellar RPC

• New CircuitBreaker class (src/lib/circuit-breaker.ts) with
  CLOSED → OPEN → HALF_OPEN states
• Configurable via RPC_BREAKER_FAILURE_THRESHOLD and
  RPC_BREAKER_RECOVERY_TIMEOUT_MS env vars
• withRpcConnection() routes through the breaker; throws
  RpcDegradedError as fallback when OPEN
• Replaced the three stub functions (isRpcAvailable,
  isRpcOutageExtended, getRpcStatus) with real implementations
• Breaker metrics exposed on GET /health; GET /ready now includes
  rpc_circuit check

#55 – Transaction retry with exponential backoff

• New withRetry() utility (src/lib/retry.ts) with configurable
  maxAttempts, baseDelayMs, maxDelayMs, and jitter
• isTransientError() prevents retries on permanent errors
  (bad auth, insufficient balance, no account, etc.)
• stellar.ts _executeSignAndSubmitWithRetry delegates to withRetry
  instead of a hand-rolled loop
• Ships missing src/lib/tx-queue.ts and src/lib/error-limiter.ts
  that index.ts already imported

#54 – Batch transaction support

• isSorobanBatchAvailable() detection stub — returns false until
  Soroban ships native batching (monitor stellar/stellar-core)
• runJob() routes to runBatchNative() or runBatchSequential() fallback
• runBatchNative() scaffolded and ready for implementation
• Per-item duration_ms tracking added to BatchResult
• Post-job benchmark summary (total_ms, avg_ms_per_item,
  throughput_per_second) attached to each completed BatchJob

Testing

• Existing test suite passes unchanged
• Manual: SIGTERM during active requests confirms clean drain before exit
• Manual: force-opening the breaker confirms /ready returns 503

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