cameras: surface per-camera REC (record-to-disk) toggle

`enabled` already gated recording only (go2rtc/live work regardless), but
it was a buried "Вкл" checkbox. Make it an explicit REC control:
- New "Запись" column with an inline REC switch in each camera row.
- Relabel the add/edit form toggle "Вкл" -> "REC" + tooltip.
- New POST /api/cameras/{id}/record {enabled} that flips recording through
  the supervisor only (no go2rtc re-sync, so live view doesn't blip).

Off = view-only (live via go2rtc); on = record segments to disk.
VERSION -> 0.0.124.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/settings.html

@@ -55,7 +55,7 @@
           </div>
           <table class="cams">
             <thead>
-              <tr><th style="width:48px">ID</th><th>Камера</th><th>IP</th><th>Потоки</th><th>Статус</th><th></th></tr>
+              <tr><th style="width:48px">ID</th><th>Камера</th><th>IP</th><th>Потоки</th><th style="width:72px">Запись</th><th>Статус</th><th></th></tr>
             </thead>
             <tbody id="cam-rows"></tbody>
           </table>