feat: Persist Vault Contract Events into Prisma

[caxtonacollins]

Persist Vault Contract Events into Prisma (Idempotent)

Closes #23

Summary

Implements idempotent vault contract event persistence with automatic deduplication and ledger cursor tracking for reliable event processing and recovery.

Changes

• Schema: Added EventCursor and ProcessedEvent models for tracking and deduplication
• Events: Implemented deposit/withdraw/rebalance event handlers with database persistence
• Idempotency: Unique constraint on (contractId, txHash, eventType, ledger) prevents duplicates
• Cursor: Persists last processed ledger for resumption on restart
• Tests: Comprehensive unit and integration tests with mocked RPC

Acceptance Criteria

✅ Deposit events mark transactions CONFIRMED and update balances
✅ Withdraw events update positions correctly
✅ Rebalance events record protocol rates
✅ No duplicate processing via deduplication logic
✅ Listener resumes from last processed ledger on restart
✅ All tests pass with proper mocking

Files Changed

• prisma/schema.prisma - Added EventCursor and ProcessedEvent models
• prisma/migrations/20260326152030_add_event_tracking/migration.sql - Migration for new tables
• src/stellar/events.ts - Full implementation of event persistence
• tests/unit/stellar/events.test.ts - Unit tests
• tests/integration/stellar/events.test.ts - Integration tests

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