feat(database): implement global transaction middleware

[mijinummi]

Database Transaction Middleware for Data Integrity (#322)

Overview

This PR introduces a global transaction middleware to ensure all database operations are atomic, consistent, isolated, and durable (ACID).

Changes

• Added TransactionManager for TypeORM transaction handling
• Implemented TransactionMiddleware to wrap requests in transactions
• Added support for isolation levels (READ COMMITTED, REPEATABLE READ, SERIALIZABLE)
• Implemented savepoints for nested transactions
• Added rollback triggers for errors, validation failures, and timeouts
• Integrated TransactionLogger for monitoring conflicts and rollbacks
• Registered middleware globally in AppModule

Acceptance Criteria Met

• ✅ Automatic transaction wrapping
• ✅ Rollback on failed requests
• ✅ Commit on success
• ✅ Deadlock and timeout handling
• ✅ Read-only transaction support
• ✅ Consistency across multi-table operations
• ✅ Logging and metrics for monitoring

Next Steps

• Add distributed transaction support for microservices
• Integrate Prometheus/Grafana metrics for transaction performance
• Extend middleware to support custom isolation per route
  Closes Database Transaction Middleware for Data Integrity #322

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