Add delete-downloaded-video and real YouTube unsubscribe

Delete local copy:
- MeTubeClient.delete_download() -> POST /delete {ids, where: "done"}
- Only ever acts on a job our own app tracked (metube_job_id we stored from
  its own 'completed' event), never a pre-existing MeTube file
- New "deleted" terminal status; DELETE /api/videos/{id}/download
- Frontend: delete button next to "На сервере" badge, confirm dialog

Unsubscribe (deliberate deviation from the original TZ's MVP exclusion of
subscription management, per explicit user request after being shown the
tradeoff):
- OAuth scope widened from youtube.readonly to full youtube (read/write) --
  existing stored tokens only cover the old scope, so unsubscribing needs a
  fresh reconnect; reads keep working unchanged on the old token meanwhile
- channels.youtube_subscription_id (distinct from the channel id; that's
  what subscriptions.delete actually keys on) captured during subscriptions
  sync
- YouTubeInsufficientScope raised on 401/403 "insufficient authentication
  scopes" and surfaced as a clear 403 asking the user to reconnect, rather
  than a generic API error
- POST /api/channels/{id}/unsubscribe calls subscriptions.delete and marks
  the channel unsubscribed locally on success
- Frontend: "Отписаться" button on ChannelCard with confirm dialog

10 new backend tests (73 total).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/App.css

@@ -127,6 +127,21 @@
   color: #999;
 }
 
+.unsubscribe-button {
+  font-size: 12px;
+  padding: 2px 8px;
+  margin-left: 8px;
+  border: 1px solid #ccc;
+  border-radius: 10px;
+  background: transparent;
+  cursor: pointer;
+}
+
+.unsubscribe-error {
+  font-size: 12px;
+  margin-top: 4px;
+}
+
 .channel-categories {
   display: flex;
   flex-wrap: wrap;
@@ -388,7 +403,8 @@
   color: #a3221f;
 }
 
-.download-error {
+.download-error,
+.download-inline-actions {
   display: inline-flex;
   align-items: center;
   gap: 6px;