feat: add event-driven backend infrastructure

[ShashankShekhar31]

Summary

Implements event-driven backend architecture for the Crypto Wallet / Exchange platform.

Included

• Added local Redpanda/Kafka infrastructure
• Added versioned event envelopes and exchange deposit events
• Implemented PostgreSQL transactional outbox
• Added outbox leasing, retry handling, and publishing worker
• Added idempotent consumer inbox
• Added dead-letter event handling
• Added Kafka event publisher and consumer
• Added Temporal OSS local infrastructure
• Added Temporal workflow/client integration
• Connected exchange deposit events:
  PostgreSQL Outbox → Redpanda/Kafka → Idempotent Consumer → Temporal Workflow
• Added exchange deposit event handler
• Added integration tests for messaging, outbox, inbox, consumer, and Temporal workflow triggering

Validation

• Typechecks passed for all affected packages
• Messaging integration tests: 20/20 passed
• git diff --cached --check: clean
• Working tree clean
• Temporal Web UI verified
• End-to-end outbox → Kafka → consumer → Temporal workflow verified
• Temporal workflows successfully completed in the local Web UI

Architecture

PostgreSQL remains the authoritative source of truth.

Application
    │
    ▼
PostgreSQL Transaction
    ├── Business State
    └── Outbox Event
            │
            ▼
       Outbox Worker
            │
            ▼
      Redpanda / Kafka
            │
            ▼
     Idempotent Consumer
            │
            ▼
      Temporal Workflow
            │
            ▼
       Workflow Activity