chore: add .env.example with required configuration templates

.env.example

@@ -0,0 +1,16 @@
+# Server Port
+PORT=3000
+
+# JWT Secret Key (generate a secure random string in production)
+JWT_SECRET=your-secret-key-here
+
+# Database URL (SQLite file path)
+# Windows format: file:C:/path/to/database.sqlite
+# Unix format: file:/path/to/database.sqlite
+DATABASE_URL=file:./data/database.sqlite
+
+# Max Request Body Size
+MAX_REQUEST_SIZE=10mb
+
+# API Base URL (for frontend)
+VITE_API_BASE_URL=/api