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/modules/realtime/services/message_service.py

@@ -5,6 +5,7 @@ from typing import List, Optional, Dict, Any
 from datetime import datetime, timedelta
 import uuid
 
+from app.core.config import get_settings
 from app.modules.realtime.models import Message, MessageType, MessageReaction, MessageEditHistory
 from app.modules.realtime.schemas import (
     MessageCreate,
@@ -13,6 +14,8 @@ from app.modules.realtime.schemas import (
     ReactionSummary
 )
 
+settings = get_settings()
+
 
 class MessageService:
     """Service for message operations"""
@@ -161,9 +164,9 @@ class MessageService:
         if message.sender_id != user_id:
             return None
 
-        # Check time limit (15 minutes)
+        # Check time limit (configurable via MESSAGE_EDIT_TIME_LIMIT_MINUTES)
         time_diff = datetime.utcnow() - message.created_at
-        if time_diff > timedelta(minutes=15):
+        if time_diff > timedelta(minutes=settings.MESSAGE_EDIT_TIME_LIMIT_MINUTES):
             return None
 
         # Store original content in edit history