feat: 引入多环境配置支持，通过 .env.dev 和 .env.prod 文件管理不同环境配置，并更新 main.go 以加载对应配置。

cmd/server/main.go

@@ -20,19 +20,37 @@ import (
 func main() {
 	// Load .env file from project root (try multiple locations)
 	envPaths := []string{
-		".env",                          // Current directory
-		"../.env",                        // Parent directory (when running from backend/)
-		"../../.env",                     // Two levels up (when running from backend/cmd/server/)
+		".env",                            // Current directory
+		"../.env",                         // Parent directory (when running from backend/)
+		"../../.env",                      // Two levels up (when running from backend/cmd/server/)
 		filepath.Join("..", "..", ".env"), // Explicit path
 	}
-	
+
+	var envDir string
 	for _, envPath := range envPaths {
 		if err := godotenv.Load(envPath); err == nil {
-			log.Printf("Loaded environment from: %s", envPath)
+			log.Printf("Loaded base environment from: %s", envPath)
+			envDir = filepath.Dir(envPath)
 			break
 		}
 	}
 
+	// Load specific environment file if APP_ENV is set (e.g. .env.dev, .env.prod)
+	// This allows switching environments by just changing APP_ENV in .env
+	if appEnv := os.Getenv("APP_ENV"); appEnv != "" {
+		targetEnvFile := ".env." + appEnv
+		targetEnvPath := targetEnvFile
+		if envDir != "" && envDir != "." {
+			targetEnvPath = filepath.Join(envDir, targetEnvFile)
+		}
+
+		if err := godotenv.Overload(targetEnvPath); err == nil {
+			log.Printf("Loaded specific environment config from: %s", targetEnvPath)
+		} else {
+			log.Printf("Note: Specific environment file %s not found or could not be loaded", targetEnvPath)
+		}
+	}
+
 	// Load configuration
 	cfg := config.Load()