Add saved channel keys feature for Web UI users

- Database: Add user_channel_keys table with CASCADE delete
- Auth: Add CRUD functions for channel key management (10 keys/user limit)
- Routes: Add key save/delete/rename endpoints and JSON API
- Account page: Add saved keys section with add/rename/delete UI
- Encode/Decode: Add saved keys to channel key dropdown (optgroup)
- About page: Add Channel Key QR generator for sharing keys
- Track last_used_at when saved keys are used

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontends/web/templates/encode.html

@@ -418,7 +418,14 @@
                             <select class="form-select" name="channel_key" id="channelSelect">
                                 <option value="auto" selected>Auto{% if channel_configured %} (Server Key){% endif %}</option>
                                 <option value="none">Public</option>
-                                <option value="custom">Custom</option>
+                                {% if saved_channel_keys %}
+                                <optgroup label="Saved Keys">
+                                    {% for key in saved_channel_keys %}
+                                    <option value="{{ key.channel_key }}" data-key-id="{{ key.id }}">{{ key.name }} ({{ key.channel_key[:4] }}...)</option>
+                                    {% endfor %}
+                                </optgroup>
+                                {% endif %}
+                                <option value="custom">Custom...</option>
                             </select>
 
                             <!-- Server channel indicator (compact) -->