feat(sharing): add document sharing with expiring links

- Add SharedLink model with token, expiry, view limit, password hash
- Add migration 025_add_shared_links
- Add API endpoints: create, list, revoke (auth) + public info/download
- Add management UI at /shared-links with revoke controls
- Add public share landing page at /share/{token}
- Add Share button on file_view.html
- Add Shared Links to user dropdown in common.js
- Write 35 unit tests covering all scenarios
- Update UserGuide.md with sharing documentation

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

frontend/templates/file_view.html

@@ -341,6 +341,9 @@
             <i class="fas fa-external-link-alt" aria-hidden="true"></i> Open original
           </a>
           {% endif %}
+          <a href="/shared-links?file_id={{ file.id }}" class="action-btn btn-secondary">
+            <i class="fas fa-share-alt" aria-hidden="true"></i> Share
+          </a>
         </div>
       </div>
     </div>