Feat: add POSIX-lite bootstrap foundation

Backend/internal/config/config.go

@@ -17,6 +17,7 @@ type Config struct {
 	APIPort         string
 	PostgresURL     string
 	ValkeyURL       string
+	POSIXRoot       string
 	ShutdownTimeout time.Duration
 }
 
@@ -29,6 +30,7 @@ func Load() *Config {
 		APIPort:         getEnv("BACKEND_API_PORT", "8081"),
 		PostgresURL:     getEnv("DATABASE_URL", "postgres://moku:moku_dev_password@localhost:5432/moku?sslmode=disable"),
 		ValkeyURL:       getEnv("VALKEY_URL", "redis://localhost:6379/0"),
+		POSIXRoot:       getEnv("POSIX_ROOT", "../POSIX"),
 		ShutdownTimeout: getDurationEnv("BACKEND_SHUTDOWN_TIMEOUT", 10*time.Second),
 	}
 }