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/main.py

@@ -9,7 +9,6 @@ from fastapi.middleware.cors import CORSMiddleware
 from fastapi.staticfiles import StaticFiles
 from fastapi.responses import FileResponse
 from app.core.config import get_settings
-from app.core.database import engine, Base
 from app.modules.auth import router as auth_router
 from app.modules.auth.users_router import router as users_router
 from app.modules.auth.middleware import auth_middleware
@@ -24,8 +23,8 @@ FRONTEND_DIR = Path(__file__).parent.parent / "frontend" / "dist"
 
 settings = get_settings()
 
-# Create database tables
-Base.metadata.create_all(bind=engine)
+# Database tables are managed by Alembic migrations
+# Run: alembic upgrade head
 
 # Initialize FastAPI app
 app = FastAPI(
@@ -35,10 +34,10 @@ app = FastAPI(
     debug=settings.DEBUG,
 )
 
-# CORS middleware (adjust for production)
+# CORS middleware - origins configured via CORS_ORIGINS environment variable
 app.add_middleware(
     CORSMiddleware,
-    allow_origins=["*"],  # TODO: Restrict in production
+    allow_origins=settings.get_cors_origins(),
     allow_credentials=True,
     allow_methods=["*"],
     allow_headers=["*"],