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/errors.json

@@ -47,7 +47,8 @@
     "source_type_invalid": "Unknown storage type",
     "query_empty": "Enter a query",
     "wave_activity_invalid": "Unknown activity",
-    "wave_mood_invalid": "Unknown mood"
+    "wave_mood_invalid": "Unknown mood",
+    "template_invalid": "Invalid path template: {{reason}}"
   },
   "users": {
     "not_found": "User not found",
@@ -56,7 +57,8 @@
   "library": {
     "source_not_found": "Storage not found",
     "scan_running": "A scan is already running",
-    "source_offline": "The storage is not connected"
+    "source_offline": "The storage is not connected",
+    "template_unavailable": "Could not suggest a template: no storage pattern converts to a template, or the storage has no files"
   },
   "catalog": {
     "not_found": "Not found"