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>

frontend/.env.example

@@ -1,11 +1,56 @@
-# Frontend Environment Variables
+# =============================================================================
+# Task Reporter - Frontend Environment Configuration
+# =============================================================================
+# Copy this file to .env and customize as needed.
+# All variables are optional and have sensible defaults for development.
+# =============================================================================
 
+# -----------------------------------------------------------------------------
+# API Configuration
+# -----------------------------------------------------------------------------
 # API Base URL (optional)
-# - For local development: leave empty or don't set (will use /api)
+# - For local development: leave empty or don't set (will use /api with proxy)
 # - For production with separate backend: set to full URL
 # Example: https://api.yourdomain.com/api
 VITE_API_BASE_URL=
 
-# Note: When set, this URL is also used for WebSocket connections
-# http:// will be converted to ws://
-# https:// will be converted to wss://
+# API request timeout in milliseconds (default: 30000 = 30 seconds)
+VITE_API_TIMEOUT_MS=30000
+
+# -----------------------------------------------------------------------------
+# Development Server Configuration
+# -----------------------------------------------------------------------------
+# Frontend development server port (default: 3000)
+VITE_PORT=3000
+
+# Backend API URL for development proxy (default: http://localhost:8000)
+# This is used by Vite's proxy configuration during development
+VITE_BACKEND_URL=http://localhost:8000
+
+# -----------------------------------------------------------------------------
+# WebSocket Configuration
+# -----------------------------------------------------------------------------
+# Maximum WebSocket reconnection delay in milliseconds (default: 30000 = 30 seconds)
+# The reconnection uses exponential backoff, capped at this value
+VITE_MAX_RECONNECT_DELAY_MS=30000
+
+# -----------------------------------------------------------------------------
+# Query/Cache Configuration
+# -----------------------------------------------------------------------------
+# Messages refetch interval in milliseconds (default: 30000 = 30 seconds)
+# Used for polling online users status
+VITE_MESSAGES_REFETCH_INTERVAL_MS=30000
+
+# Reports stale time in milliseconds (default: 30000 = 30 seconds)
+# Time before cached report data is considered stale
+VITE_REPORTS_STALE_TIME_MS=30000
+
+# =============================================================================
+# Notes
+# =============================================================================
+# - All VITE_ prefixed variables are exposed to the browser
+# - Never put sensitive data (API keys, secrets) in frontend environment variables
+# - When VITE_API_BASE_URL is set:
+#   - http:// URLs will be converted to ws:// for WebSocket connections
+#   - https:// URLs will be converted to wss:// for WebSocket connections
+# =============================================================================