feat: off-chain sync and async task infrastructure

[aji70]

Off-chain sync and async task infrastructure

Ensures the off-chain database stays in sync with on-chain reality and handles asynchronous tasks (email/notifications).

Changes

EventBridgeService

• Polls Soroban events and keeps DB state in sync
• Handles PaymentSettled, PaymentFailed, PaymentInitiated
• Configurable poll interval and error backoff (EVENT_BRIDGE_POLL_MS, EVENT_BRIDGE_ERROR_BACKOFF_MS)
• Optional contract ID filtering via SOROBAN_CONTRACT_IDS
• Deduplication and graceful shutdown

QueueService

• BullMQ + Redis for job processing
• Structured job payloads for Email, Notification, Sync, BlockchainTx
• Typed add*Job methods and shared retry/backoff options
• Queue name: zaps:jobs
  closes [WORKERS] On-Chain Event Bridge and Background Jobs #62

JobProcessors

• EmailProcessor – email send flow (SendGrid/SES integration-ready)
• NotificationProcessor – stores notifications in DB and enqueues push (FCM-ready)
• SyncProcessor – user_data, analytics, backup, on_chain_sync
• BlockchainTxProcessor – simulates and submits XDR, updates Payment/Transfer status

All processors use structured logging and consistent error handling.

JobProcessorRegistry

• Central mapping of job types to processor functions
• Single worker entry point that resolves the processor via registry

Worker configuration

• Configurable concurrency (WORKER_CONCURRENCY)
• Retries with exponential backoff (JOB_MAX_RETRIES, JOB_BACKOFF_DELAY_MS)
• Graceful shutdown on SIGINT/SIGTERM

New files

• server/src/config/worker.config.ts
• server/src/types/job-payloads.ts
• server/src/processors/ – email, notification, sync, blockchain-tx, index (registry)
• server/src/utils/soroban-events.ts

Environment variables

See server/.env.example for:

• SOROBAN_CONTRACT_IDS
• EVENT_BRIDGE_POLL_MS, EVENT_BRIDGE_ERROR_BACKOFF_MS
• WORKER_CONCURRENCY, JOB_MAX_RETRIES, JOB_BACKOFF_DELAY_MS