feat(database): implement transaction middleware with ACID

[mijinummi]

🚀 #322 Database Transaction Middleware for Data Integrity

📌 Overview

This PR introduces a robust transaction middleware layer to enforce ACID guarantees across all database operations. It ensures that every request is executed within a controlled transactional context, preventing partial writes, data inconsistencies, and connection leaks.

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🎯 Key Features

✅ Automatic Transaction Management

• All database operations are wrapped in a transaction by default
• Transactions are committed on success and rolled back on failure

🔐 Isolation Levels Support

• READ COMMITTED (default)
• REPEATABLE READ (critical operations)
• SERIALIZABLE (leaderboards, competitions)

🔁 Nested Transactions (Savepoints)

• Supports nested transactional flows using savepoints
• Prevents partial failures in complex service orchestration

⏱ Timeout & Deadlock Handling

• Configurable transaction timeout
• Automatic rollback on timeout
• Deadlock detection and graceful retry/failure handling

📖 Read-Only Transactions

• Optimized transactions for read-heavy endpoints
• Reduces locking and improves performance

🧠 Manual Transaction Control

• Allows overriding default behavior for advanced use cases

📊 Logging & Observability

• Logs:
  • transaction start/end
  • rollbacks
  • deadlocks
  • timeouts
• Enables future integration with monitoring systems

🔌 TypeORM Integration

• Uses QueryRunner for full lifecycle control
• Ensures proper connection release (no leaks)

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🧪 Covered Scenarios

Scenario	Behavior
Puzzle submission	Atomic update of progress, XP, streak
User registration	User + streak + email handled transactionally
Points claiming	XP update + transaction log + achievements
Friend request	Relationship + notification consistency

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⚠️ Rollback Triggers

• Validation errors
• Database constraint violations
• Business logic failures
• External API failures (critical paths)
• Transaction timeout exceeded

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🧩 Edge Case Handling

• Deadlock resolution strategy implemented
• Nested transaction conflict safety
• Connection pool exhaustion protection
• Safe cleanup on all error paths

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✅ Acceptance Criteria Checklist

• All DB operations are transactional by default
• Failed requests rollback completely
• Successful requests commit atomically
• Deadlocks handled gracefully
• Transaction timeouts enforced
• Read-only optimization supported
• Multi-entity operations remain consistent
• No connection leaks observed
• Logging and metrics implemented

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Closes #322