feat(views): add document-centric view at /files/{id}

- Revert search.html and files.html links back to /files/{id}
- Add GET /files/{file_id} route (file_view_page) with workdir path-
  containment guards (os.path.commonpath) to prevent traversal
- Create file_view.html: document-centric page showing AI metadata,
  inline PDF preview, extracted OCR text, download actions, file info,
  status pill, and link to /files/{id}/detail for process pipeline view
- Existing /files/{id}/detail route is unchanged

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

frontend/templates/files.html

@@ -1012,7 +1012,7 @@
             ${snippet ? `<div style="margin-top: 0.4rem; font-size: 0.8rem; color: #374151; white-space: pre-wrap; word-break: break-word;">…${snippet}…</div>` : ''}
           </div>
           <div style="flex-shrink: 0;">
-            <a href="/files/${hit.file_id}/detail" style="padding: 0.25rem 0.6rem; background: #f3f4f6; color: #374151; border-radius: 0.25rem; font-size: 0.8rem; text-decoration: none; white-space: nowrap;" title="View file">
+            <a href="/files/${hit.file_id}" style="padding: 0.25rem 0.6rem; background: #f3f4f6; color: #374151; border-radius: 0.25rem; font-size: 0.8rem; text-decoration: none; white-space: nowrap;" title="View file">
               <i class="fas fa-external-link-alt"></i>
             </a>
           </div>