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/static/js/common.js

@@ -212,6 +212,9 @@ function _makeMenuLink(href, iconClass, label, extraClasses = '') {
         linksDiv.appendChild(
           _makeMenuLink('/api-tokens', 'fas fa-key text-yellow-500', 'API Tokens', 'text-gray-700')
         );
+        linksDiv.appendChild(
+          _makeMenuLink('/shared-links', 'fas fa-share-alt text-blue-400', 'Shared Links', 'text-gray-700')
+        );
 
         // Divider + Sign Out
         const divider = document.createElement('div');
@@ -293,6 +296,18 @@ function _makeMenuLink(href, iconClass, label, extraClasses = '') {
         tokensLink.appendChild(document.createTextNode('API Tokens'));
         mobileAuthSection.appendChild(tokensLink);
 
+        // Shared Links link
+        const sharedLinksLink = document.createElement('a');
+        sharedLinksLink.href = '/shared-links';
+        sharedLinksLink.className =
+          'flex items-center px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50';
+        const sharedLinksIcon = document.createElement('i');
+        sharedLinksIcon.className = 'fas fa-share-alt mr-2 text-blue-400';
+        sharedLinksIcon.setAttribute('aria-hidden', 'true');
+        sharedLinksLink.appendChild(sharedLinksIcon);
+        sharedLinksLink.appendChild(document.createTextNode('Shared Links'));
+        mobileAuthSection.appendChild(sharedLinksLink);
+
         // Logout link
         const logoutLink = document.createElement('a');
         logoutLink.href = '/logout';