Simplify database support to SQLite only

Removed PostgreSQL and MySQL support to reduce complexity:
- Removed postgres and mysql drivers from go.mod
- Simplified database.Initialize() to only use SQLite
- Simplified DatabaseConfig struct (removed Driver, Host, Port, User, Password, SSLMode)
- Removed GetDSN() method from config
- Removed --db CLI flag (only --db-name remains for specifying SQLite file path)
- Updated .env.example to remove MySQL/PostgreSQL options
- Updated README_GOLANG.md to reflect SQLite-only support
- Ran go mod tidy to clean up dependencies

This makes the application simpler to deploy and maintain, with no
external database dependencies required.

.env.example

@@ -3,27 +3,9 @@ SERVER_HOST=0.0.0.0
 SERVER_PORT=3000
 GIN_MODE=debug
 
-# Database Configuration
-DB_DRIVER=sqlite
+# Database Configuration (SQLite)
 DB_NAME=tracks.db
 
-# For PostgreSQL:
-# DB_DRIVER=postgres
-# DB_HOST=localhost
-# DB_PORT=5432
-# DB_NAME=tracks
-# DB_USER=tracks
-# DB_PASSWORD=tracks
-# DB_SSLMODE=disable
-
-# For MySQL:
-# DB_DRIVER=mysql
-# DB_HOST=localhost
-# DB_PORT=3306
-# DB_NAME=tracks
-# DB_USER=tracks
-# DB_PASSWORD=tracks
-
 # Authentication Configuration
 JWT_SECRET=your-secret-key-change-this-in-production
 TOKEN_EXPIRY_HOURS=168