feat(schema): 데이터베이스 스키마 강화 마이그레이션

데이터 무결성:
- user_deposits.balance >= 0 CHECK 제약조건
- deposit_transactions.depositor_name 최대 50자 제한
- 음수 잔액 방지, 긴 이름 방지

감사 추적:
- audit_logs 테이블 생성
- 모든 중요 작업 추적 (user_id, action, resource, details)
- 인덱스 추가 (user_id, action, created_at)

프로덕션 안전:
- 백업 → 재생성 → 복원 방식
- 롤백 스크립트 포함
- 데이터 유실 방지 로직
- 음수 잔액 데이터 감지 및 로그

마이그레이션 파일:
- migrations/001_schema_enhancements.sql (5.5K)
- migrations/001_rollback.sql (4.0K)
- migrations/AUDIT_LOG_EXAMPLES.ts (11K)
- migrations/TEST_RESULTS.md (8.0K)
- migrations/README.md (2.8K)

문서:
- SCHEMA_MIGRATION_GUIDE.md (13K) - 완전한 배포 가이드
- MIGRATION_SUMMARY.md (9.1K) - 요약 및 체크리스트

로컬 테스트 결과:
- ✅ 마이그레이션 성공 (23 commands, <1초)
- ✅ CHECK 제약조건 작동 (음수 잔액 거부)
- ✅ 길이 제한 작동 (51자 이름 거부)
- ✅ audit_logs 테이블 정상
- ✅ 데이터 보존 확인 (users:3, deposits:1, transactions:1)
- ✅ 음수 잔액 데이터 감지 (user_id:3, balance:-500)

프로덕션 배포:
- 로컬 테스트 완료, 프로덕션 준비 완료
- 배포 전 백업 필수
- 예상 소요 시간: <5분

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

migrations/README.md

@@ -0,0 +1,101 @@
+# Database Migrations
+
+This directory contains database migration scripts for the Telegram Bot Workers project.
+
+## Files
+
+| File | Purpose |
+|------|---------|
+| `001_schema_enhancements.sql` | Migration script - adds CHECK constraints and audit logging |
+| `001_rollback.sql` | Rollback script - reverts migration 001 |
+| `AUDIT_LOG_EXAMPLES.ts` | TypeScript examples for using audit logs |
+| `TEST_RESULTS.md` | Local test results and verification |
+| `README.md` | This file |
+
+## Quick Start
+
+### Local Testing
+
+```bash
+# Initialize local database
+npm run db:init:local
+
+# Add test data
+wrangler d1 execute telegram-conversations --local \
+  --command "INSERT INTO users (telegram_id, username) VALUES ('123', 'test')"
+
+# Run migration
+wrangler d1 execute telegram-conversations --local \
+  --file migrations/001_schema_enhancements.sql
+
+# Verify constraints work
+wrangler d1 execute telegram-conversations --local \
+  --command "INSERT INTO user_deposits (user_id, balance) VALUES (999, -1000)"
+# Expected: CHECK constraint failed
+```
+
+### Production Deployment
+
+**⚠️ MANDATORY: Read SCHEMA_MIGRATION_GUIDE.md first**
+
+```bash
+# 1. Backup database
+wrangler d1 execute telegram-conversations \
+  --command ".dump" > backup_$(date +%Y%m%d_%H%M%S).sql
+
+# 2. Run migration
+wrangler d1 execute telegram-conversations \
+  --file migrations/001_schema_enhancements.sql
+
+# 3. Verify success
+wrangler d1 execute telegram-conversations \
+  --command "SELECT 'Migration completed' as status, datetime('now') as timestamp"
+```
+
+## Migration 001: Schema Enhancements
+
+**Date**: 2026-01-19
+
+### Changes
+
+1. **user_deposits**: Add `balance >= 0` CHECK constraint
+2. **deposit_transactions**: Add `depositor_name` length <= 50 CHECK constraint
+3. **audit_logs**: Create new table for operation tracking
+
+### Benefits
+
+- Prevent negative balances at database level
+- Enforce depositor name length limit
+- Track all critical operations for compliance and debugging
+- Improved data integrity and auditability
+
+### Risk Level
+
+- **Low**: No breaking changes to application code
+- **No downtime**: Migration completes in < 1 second for typical datasets
+- **Reversible**: Rollback script available
+
+### Files
+
+- Migration: `001_schema_enhancements.sql`
+- Rollback: `001_rollback.sql`
+- Guide: `../SCHEMA_MIGRATION_GUIDE.md`
+- Examples: `AUDIT_LOG_EXAMPLES.ts`
+- Tests: `TEST_RESULTS.md`
+
+## Documentation
+
+For detailed information, see:
+
+- **SCHEMA_MIGRATION_GUIDE.md**: Complete deployment guide
+- **TEST_RESULTS.md**: Local test results and verification
+- **AUDIT_LOG_EXAMPLES.ts**: Usage examples for audit logs
+
+## Support
+
+If issues occur during migration:
+
+1. Check logs: `wrangler tail`
+2. Review SCHEMA_MIGRATION_GUIDE.md troubleshooting section
+3. Rollback if necessary: `001_rollback.sql`
+4. Restore from backup if critical failure