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>

apps/web/src/pages/admin/SourcesPage.tsx

@@ -110,6 +110,7 @@ function SourceCard({ source, onEdit }: { source: Source; onEdit: () => void })
           {source.name}
           <span className={l.typeBadge}>{t(`sources.type_${source.type}`)}</span>
           <StatusBadge status={source.status} />
+          {source.accepts_uploads && <span className={l.typeBadge}>{t('write_template.badge')}</span>}
         </div>
         <span className={l.location}>{location(source)}</span>
         {source.status === 'error' && source.status_message && (
@@ -140,6 +141,11 @@ function SourceCard({ source, onEdit }: { source: Source; onEdit: () => void })
             {test.ok ? t('sources.test_ok', { count: test.audio_files }) : t('sources.test_fail', { message: test.message })}
           </span>
         )}
+        {test && test.write_ok !== null && test.write_ok !== undefined && (
+          <span className={`${l.test} ${test.write_ok ? l.testOk : l.testFail}`} role="status">
+            {test.write_ok ? t('write_template.write_ok') : t('write_template.write_fail', { message: test.write_message ?? '' })}
+          </span>
+        )}
         <FormError>{error ? errorText(error) : null}</FormError>
       </div>
       {confirmDelete && (