feat: add event worker deduplication and request ID tracing

[WISDOM-WOKE22]

Summary

Implements two critical hardening features to improve system reliability and observability:

1. Event Worker Deduplication (Addresses #383)

• Adds unique constraint on StreamEvent(transactionHash, eventType) to prevent duplicate event processing
• Replaces cursor-based pagination vulnerability where crashes between DB write and cursor update caused duplicate records
• Implements idempotent event processing with upsert pattern and warning logs for detected duplicates
• New test validates replay safety: processing same batch twice produces one record

2. Request ID Tracing (Addresses #381)

• Adds end-to-end request correlation via AsyncLocalStorage and UUID generation
• Every log entry automatically includes requestId field for trace correlation
• Response includes X-Request-ID header (echoes client-provided ID if valid, up to 128 chars)
• SSE connected event includes requestId for client-side connection tracking
• Logs request received/response sent with method, path, status, duration, and requestId

Changes

Backend Schema & Migrations:

• backend/prisma/schema.prisma — Added unique constraint
• backend/prisma/migrations/20260428000000_add_stream_event_unique_constraint/ — Migration SQL

Event Worker Deduplication:

• backend/src/workers/soroban-event-worker.ts — Updated 6 event handlers (CREATED, TOPPED_UP, WITHDRAWN, CANCELLED, COMPLETED, FEE_COLLECTED) to use upsert pattern with duplicate detection
• backend/tests/soroban-event-worker.test.ts — New test suite validating idempotency

Request ID Middleware:

• backend/src/middleware/requestId.ts — New middleware (AsyncLocalStorage + logging)
• backend/src/logger.ts — Added requestContext export and Winston format enhancement
• backend/src/app.ts — Mounted requestId middleware in request chain
• backend/src/controllers/sse.controller.ts — Added requestId to SSE connected event

Test Results

✅ All event worker idempotency tests pass
✅ All previously passing tests still pass
✅ Zero impact on existing functionality

closes #383, closes #381

🤖 Generated with Claude Code

[ogazboiz]

hey, main was failing CI from broken auth imports + frontend parse errors. fixed and pushed to main now. please rebase to pick up the fixes:

git fetch upstream
git rebase upstream/main
git push --force-with-lease

if there's a conflict, resolve it locally and we'll review once CI is green.