feat: Migrate to MySQL and add unified environment configuration

## Database Migration (SQLite → MySQL)
- Add Alembic migration framework
- Add 'tr_' prefix to all tables to avoid conflicts in shared database
- Remove SQLite support, use MySQL exclusively
- Add pymysql driver dependency
- Change ad_token column to Text type for long JWT tokens

## Unified Environment Configuration
- Centralize all hardcoded settings to environment variables
- Backend: Extend Settings class in app/core/config.py
- Frontend: Use Vite environment variables (import.meta.env)
- Docker: Move credentials to environment variables
- Update .env.example files with comprehensive documentation

## Test Organization
- Move root-level test files to tests/ directory:
  - test_chat_room.py → tests/test_chat_room.py
  - test_websocket.py → tests/test_websocket.py
  - test_realtime_implementation.py → tests/test_realtime_implementation.py
- Fix path references in test_realtime_implementation.py

Breaking Changes:
- CORS now requires explicit origins (no more wildcard)
- All database tables renamed with 'tr_' prefix
- SQLite no longer supported

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/core/database.py

@@ -1,4 +1,8 @@
-"""Database connection and session management"""
+"""Database connection and session management
+
+Supports MySQL database with connection pooling.
+All tables use 'tr_' prefix to avoid conflicts in shared database.
+"""
 from sqlalchemy import create_engine
 from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy.orm import sessionmaker
@@ -6,10 +10,13 @@ from app.core.config import get_settings
 
 settings = get_settings()
 
-# Create engine
+# Create engine with MySQL connection pooling
 engine = create_engine(
     settings.DATABASE_URL,
-    connect_args={"check_same_thread": False} if "sqlite" in settings.DATABASE_URL else {},
+    pool_size=5,
+    max_overflow=10,
+    pool_pre_ping=True,  # Verify connection before using
+    pool_recycle=3600,  # Recycle connections after 1 hour
     echo=settings.DEBUG,
 )