Update to use MySQL Database

cmd/server/main.go

@@ -11,7 +11,14 @@ import (
 )
 
 func main() {
-	dsn := getenv("DATABASE_DSN", "walkies.db")
+	// Build MySQL DSN from environment variables
+	dbHost := getenv("DB_HOST", "localhost")
+	dbPort := getenv("DB_PORT", "3306")
+	dbUser := getenv("DB_USER", "root")
+	dbPassword := getenv("DB_PASSWORD", "")
+	dbName := getenv("DB_NAME", "walkies")
+	dsn := fmt.Sprintf("%s:%s@tcp(%s:%s)/%s?parseTime=true", dbUser, dbPassword, dbHost, dbPort, dbName)
+
 	jwtSecret := getenv("JWT_SECRET", "change-me-in-production")
 	staticDir := getenv("STATIC_DIR", "./web/dist")
 	port := getenv("PORT", "8080")