Implement comprehensive bookmark/save system

Added full bookmark functionality for users to save posts:

Backend Features:
- New Bookmark model with user_id, post_uuid, and cached metadata
- API endpoints:
  - POST /api/bookmark - Toggle bookmark status
  - GET /api/bookmarks - Get user's bookmarks with pagination
  - GET /api/bookmark-status/<uuid> - Check if post is bookmarked
- Database migration script for bookmarks table

Frontend Features:
- New /bookmarks page with post list and management
- Bookmark buttons on post cards with save/unsave toggle
- Real-time bookmark status loading and updates
- Navigation menu integration
- Responsive design with archived post handling

UI Components:
- Modern bookmark button with hover states
- Pagination for bookmark listings
- Empty state for users with no bookmarks
- Error handling and loading states
- Remove bookmark functionality

The system handles:
- Unique bookmarks per user/post combination
- Cached post metadata for performance
- Graceful handling of deleted/archived posts
- Authentication requirements for all bookmark features

Users can now save posts for later reading and manage their bookmarks
through a dedicated bookmarks page.

Fixes #20

~claude

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

templates/_nav.html

@@ -30,6 +30,7 @@
                     </div>
                     <div class="user-dropdown">
                         <a href="{{ url_for('settings') }}" class="dropdown-item">⚙️ Settings</a>
+                        <a href="{{ url_for('bookmarks') }}" class="dropdown-item">📚 Bookmarks</a>
                         {% if current_user.is_admin %}
                             <a href="{{ url_for('admin_panel') }}" class="dropdown-item">👨‍💼 Admin Panel</a>
                         {% endif %}