feat(auth): add admin user management dashboard

- Add UserProfile model (app/models.py) with per-user settings: display_name, daily_upload_limit, notes, is_blocked
- Add Alembic migration 013_add_user_profiles for the new table
- Add REST API at /api/admin/users/ with list, get, upsert (PUT), delete endpoints (admin-only)
- Add HTML template admin_users.html with Alpine.js: filterable user list, paginated table, edit/create modal, delete confirmation modal
- Add view handler at /admin/users (admin-only redirect guard)
- Register routers in app/api/__init__.py and app/views/__init__.py
- Add 'Users' link to admin nav dropdown in base.html (desktop + mobile)
- Add 27 tests covering auth, list, get, upsert, delete, and model constraints
- Register UserProfile in conftest.py model imports
- Document new endpoints in docs/API.md

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

frontend/templates/base.html

@@ -95,6 +95,9 @@
                   <a href="/settings" role="menuitem" class="flex items-center px-4 py-2 text-sm text-gray-700 hover:bg-gray-100">
                     <i class="fas fa-cog w-4 mr-2 text-gray-500" aria-hidden="true"></i> Settings
                   </a>
+                  <a href="/admin/users" role="menuitem" class="flex items-center px-4 py-2 text-sm text-gray-700 hover:bg-gray-100">
+                    <i class="fas fa-users w-4 mr-2 text-blue-500" aria-hidden="true"></i> Users
+                  </a>
                   <a href="/admin/credentials" role="menuitem" class="flex items-center px-4 py-2 text-sm text-gray-700 hover:bg-gray-100">
                     <i class="fas fa-key w-4 mr-2 text-yellow-500" aria-hidden="true"></i> Credentials
                   </a>
@@ -178,6 +181,9 @@
                   <a href="/settings" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">
                     <i class="fas fa-cog mr-2 text-gray-400" aria-hidden="true"></i> Settings
                   </a>
+                  <a href="/admin/users" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">
+                    <i class="fas fa-users mr-2 text-blue-400" aria-hidden="true"></i> Users
+                  </a>
                   <a href="/admin/credentials" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">
                     <i class="fas fa-key mr-2 text-yellow-400" aria-hidden="true"></i> Credentials
                   </a>