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>

tests/test_sync.py

@@ -13,8 +13,20 @@ def test_sync_subscriptions_idempotent_and_unsubscribes(monkeypatch, db_session)
         sync.youtube_client,
         "fetch_subscriptions",
         lambda creds: [
-            {"youtube_channel_id": "chanA", "title": "Channel A", "description": "d", "thumbnail_url": "t"},
-            {"youtube_channel_id": "chanB", "title": "Channel B", "description": "d", "thumbnail_url": "t"},
+            {
+                "youtube_channel_id": "chanA",
+                "youtube_subscription_id": "subA",
+                "title": "Channel A",
+                "description": "d",
+                "thumbnail_url": "t",
+            },
+            {
+                "youtube_channel_id": "chanB",
+                "youtube_subscription_id": "subB",
+                "title": "Channel B",
+                "description": "d",
+                "thumbnail_url": "t",
+            },
         ],
     )
     monkeypatch.setattr(
@@ -39,8 +51,20 @@ def test_sync_subscriptions_idempotent_and_unsubscribes(monkeypatch, db_session)
         sync.youtube_client,
         "fetch_subscriptions",
         lambda creds: [
-            {"youtube_channel_id": "chanB", "title": "Channel B", "description": "d", "thumbnail_url": "t"},
-            {"youtube_channel_id": "chanC", "title": "Channel C", "description": "d", "thumbnail_url": "t"},
+            {
+                "youtube_channel_id": "chanB",
+                "youtube_subscription_id": "subB",
+                "title": "Channel B",
+                "description": "d",
+                "thumbnail_url": "t",
+            },
+            {
+                "youtube_channel_id": "chanC",
+                "youtube_subscription_id": "subC",
+                "title": "Channel C",
+                "description": "d",
+                "thumbnail_url": "t",
+            },
         ],
     )