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/PatternsPage.tsx

@@ -11,6 +11,7 @@ import { addIcon, arrowDownIcon, arrowUpIcon, deleteIcon } from '../../shared/ui
 import { Button, cx, Field, FormError, Icon } from '../../shared/ui/ui';
 import s from '../page.module.css';
 import l from './library.module.css';
+import { WriteTemplateSection } from './WriteTemplateSection';
 
 type Pattern = components['schemas']['PathPattern'];
 type Preview = components['schemas']['PatternPreview'];
@@ -155,6 +156,7 @@ export function PatternsPage({ id }: { id: string }) {
         <FormError>{err && !perr ? errorText(err) : null}</FormError>
       </section>
       {preview.data && <PreviewTable preview={preview.data} />}
+      {source.data && <WriteTemplateSection source={source.data} />}
     </div>
   );
 }