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>

packages/i18n/locales/en/admin.json

@@ -159,5 +159,29 @@
     "wave_listen": "Played ≥ 50%",
     "wave_early_skip": "Early skips",
     "wave_likes": "Likes per 100"
+  },
+  "write_template": {
+    "title": "Uploads and path template",
+    "intro": "Approved tracks from the upload queue are placed here using a template that repeats this storage's folder layout.",
+    "accepts": "Accept uploads into this storage",
+    "nfs_rw_hint": "The storage will be reconnected: the agent also mounts it for writing (visible only to the agent). The NFS export must allow writes for PIPU_FILE_UID.",
+    "template": "Path template",
+    "hint": "For example {albumartist}/{year} - {album}/{disc:1-}{track:02} - {title}.{ext}; [ … ] is an optional part",
+    "suggest": "Suggest from layout",
+    "check": "Check",
+    "save": "Save",
+    "saved": "Saved",
+    "required": "Set a path template to accept uploads.",
+    "no_tracks": "The storage has no tracks yet, so there is nothing to check the template against.",
+    "check_ok": "The template is reversible: {{round_trip}} of {{total}} paths parse back unchanged, {{same_path}} match the real ones.",
+    "check_warn": "Mismatches found: {{round_trip}} of {{total}} parse back unchanged, {{same_path}} match the real paths (90% needed). New files may be laid out differently from existing ones.",
+    "col_real": "Real path",
+    "col_rendered": "From template",
+    "col_problem": "Mismatch",
+    "no_pattern": "no pattern matched",
+    "other_path": "different path",
+    "badge": "accepts uploads",
+    "write_ok": "Write: ok",
+    "write_fail": "Write failed: {{message}}"
   }
 }