feat: storage writes via storage-agent, accepts_uploads and write_template (M5.5)

- storage-agent: POST /internal/files copies a staged file into a source
  (temp file, fsync, SHA-256 check, hard link so nothing is overwritten,
  " (2)" suffix on conflict, owner PIPU_FILE_UID/GID) and
  POST /internal/files/test. NFS sources that accept uploads are mounted
  rw in an agent-only /library-rw and bind-mounted read-only for others;
  local sources are written through /library-host-rw.
- meta: write templates ({track:02}, {disc:1-}, [optional]) with name
  sanitising and length limits, suggestion from the source's regexes
  and a reversibility check against 50 catalogue tracks.
- API and admin UI: accepts_uploads / write_template on sources,
  write-template suggest and check endpoints, write test result.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

services/api/internal/platform/config/config.go

@@ -66,9 +66,16 @@ type API struct {
 
 // StorageAgent — конфигурация внутреннего сервиса storage-agent.
 type StorageAgent struct {
-	HTTPAddr        string        `env:"HTTP_ADDR" envDefault:":8090"`
-	InternalToken   string        `env:"INTERNAL_TOKEN,required,notEmpty"`
-	LibraryRoot     string        `env:"LIBRARY_ROOT" envDefault:"/library"`
+	HTTPAddr      string `env:"HTTP_ADDR" envDefault:":8090"`
+	InternalToken string `env:"INTERNAL_TOKEN,required,notEmpty"`
+	LibraryRoot   string `env:"LIBRARY_ROOT" envDefault:"/library"`
+	// Запись импортированных загрузок: rw-точки NFS (только внутри агента), LIBRARY_HOST на запись, staging.
+	LibraryRWRoot string `env:"LIBRARY_RW_ROOT" envDefault:"/library-rw"`
+	HostRWRoot    string `env:"LIBRARY_HOST_RW_ROOT" envDefault:"/library-host-rw"`
+	UploadsRoot   string `env:"UPLOADS_ROOT" envDefault:"/uploads"`
+	// Владелец новых файлов на файловом сервере; -1 — оставить как есть.
+	FileUID         int           `env:"PIPU_FILE_UID" envDefault:"-1"`
+	FileGID         int           `env:"PIPU_FILE_GID" envDefault:"-1"`
 	LogLevel        string        `env:"LOG_LEVEL" envDefault:"info"`
 	ShutdownTimeout time.Duration `env:"SHUTDOWN_TIMEOUT" envDefault:"15s"`
 }