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/config.go

@@ -4,8 +4,11 @@ import (
 	"fmt"
 	"os"
 	"path"
+	"strconv"
 )
 
+const envPrefix = "AETHERA_"
+
 type cliParams struct {
 	ListenAddr   string
 	ListenPort   int
@@ -13,6 +16,24 @@ type cliParams struct {
 	SettingsFile string
 }
 
+// getEnvOrDefault returns the value of an environment variable or a default value
+func getEnvOrDefault(key, defaultValue string) string {
+	if value := os.Getenv(envPrefix + key); value != "" {
+		return value
+	}
+	return defaultValue
+}
+
+// getEnvIntOrDefault returns the integer value of an environment variable or a default value
+func getEnvIntOrDefault(key string, defaultValue int) int {
+	if value := os.Getenv(envPrefix + key); value != "" {
+		if intVal, err := strconv.Atoi(value); err == nil {
+			return intVal
+		}
+	}
+	return defaultValue
+}
+
 func (p *cliParams) Validate() error {
 	// Ensure Generated Directories
 	imgDir := path.Join(p.DataDir, "generated/images")