docs: Add `REFACTORING.md` to document applied refactoring patterns and improvements.

REFACTORING.md

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+# Refactoring History—MediaManager Core
+
+This document records the refactorings applied during development.
+
+## Refactoring #1: Connection Pool Implementation
+
+**Date**: 2025-11-13 
+**Commits**: ```22476abb27d95e88ae0439d97ef97467b6a4f985```
+
+### Problem Identified
+- Database connections were created multiple times
+- No centralized control
+- Possible resource leaks
+
+### Technique Applied
+- **Pattern**: Singleton Pattern
+- **Refactoring**: Extract Class + Introduce Singleton
+
+### Solution
+Created `DatabaseManager` class that:
+- Maintains single instance (Singleton)
+- Manages connection pool
+- Provides centralized `getConnection()` method
+
+### Result
+- ✅ Single reusable connection
+- ✅ Code centralized in one class
+- ✅ Thread-safe
+- ✅ Easier to test
+
+### Extracted Component
+`DatabaseManager` - available in `mediamanager-components`
+
+---
+
+## Refactoring #2: Thread Pool for IPC
+
+**Date**: 2025-11-14  
+**Commits**: ```f1071dca03da33f7355a6927497509b5aedaa4a2```
+
+### Problem Identified
+- New thread created for each IPC client
+- Doesn't scale well
+- Thread creation overhead
+
+### Technique Applied
+- **Pattern**: Thread Pool Pattern
+- **Refactoring**: Replace Thread Creation with Thread Pool
+
+### Solution
+Implemented `ExecutorService` with:
+- Fixed thread pool
+- Thread reuse
+- Ordered shutdown
+
+### Result
+- ✅ Better performance
+- ✅ Controlled resource usage
+- ✅ Scalability
+
+### Extracted Component
+`IPCServer` - available in `mediamanager-components`
+
+---
+
+## Refactoring #3: Graceful Shutdown
+
+**Date**: 2025-11-13  
+**Commits**: ```65e14930076db71a312feae373d6c5daee3c4d29```,```b7ff9d1e16c85bf986a770676827bee390f0d53c``` 
+
+### Problem Identified
+- Resources weren't released on exit
+- Database connections remained open
+- Thread pool didn't terminate correctly
+
+### Technique Applied
+- **Pattern**: Shutdown Hook Pattern
+- **Refactoring**: Introduce Shutdown Protocol
+
+### Solution
+Implemented shutdown hooks that:
+- Close database connections
+- Terminate thread pools
+- Release sockets
+- In specific order with timeouts
+
+### Result
+- ✅ Ordered cleanup
+- ✅ No resource leaks
+- ✅ Shutdown logging
+- ✅ Timeout to prevent hanging
+
+### Extracted Component
+Logic incorporated in `DatabaseManager` and `IPCServer`
+
+---
+
+## Applied Principles
+
+- **DRY** (Don't Repeat Yourself): Elimination of duplicated code
+- **SOLID**: Especially Single Responsibility Principle
+- **Design Patterns**: Singleton, Thread Pool, Shutdown Hook
+- **Clean Code**: Clear names, small methods, well-defined responsibilities