feat(backend): add environment variable configuration support

Add AETHERA_ prefixed env vars for server configuration:
- AETHERA_DATA_DIR: data directory path
- AETHERA_LISTEN: listen address
- AETHERA_PORT: listen port

Env vars take precedence over defaults but CLI flags override both.

backend/cmd/main.go

@@ -13,17 +13,17 @@ import (
 
 var (
 	params = cliParams{
-		ListenAddr: "localhost",
-		ListenPort: 8080,
-		DataDir:    "./data",
+		ListenAddr: getEnvOrDefault("LISTEN", "localhost"),
+		ListenPort: getEnvIntOrDefault("PORT", 8080),
+		DataDir:    getEnvOrDefault("DATA_DIR", "./data"),
 	}
 	rootCmd = &cobra.Command{Use: "aethera"}
 )
 
 func init() {
-	rootCmd.PersistentFlags().StringVar(&params.DataDir, "data-dir", "data", "Directory to store generated images")
-	rootCmd.PersistentFlags().StringVar(&params.ListenAddr, "listen", "localhost", "Address to listen on")
-	rootCmd.PersistentFlags().IntVar(&params.ListenPort, "port", 8080, "Port to listen on")
+	rootCmd.PersistentFlags().StringVar(&params.DataDir, "data-dir", params.DataDir, "Directory to store generated images (env: AETHERA_DATA_DIR)")
+	rootCmd.PersistentFlags().StringVar(&params.ListenAddr, "listen", params.ListenAddr, "Address to listen on (env: AETHERA_LISTEN)")
+	rootCmd.PersistentFlags().IntVar(&params.ListenPort, "port", params.ListenPort, "Port to listen on (env: AETHERA_PORT)")
 }
 
 func main() {