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>

deploy/.env.example

@@ -7,6 +7,10 @@ PIPU_DATA=/srv/pipu
 # Папка с музыкой на локальном диске сервера (источники type=local), например смонтированный диск.
 # Пусто — $PIPU_DATA/library-host. NFS-хранилища сюда не относятся: их подключает админка.
 LIBRARY_HOST=
+# Владелец файлов, которые импорт загрузок пишет в хранилища (UID/GID владельца музыки на файловом сервере).
+# -1 — не менять (файлы будут принадлежать root агента).
+PIPU_FILE_UID=-1
+PIPU_FILE_GID=-1
 
 # --- Домен и HTTPS (docs/install-guide.md) ---
 # Вариант А: встроенный Caddy с Let's Encrypt — оставьте строку COMPOSE_FILE и заполните DOMAIN, ACME_EMAIL.