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>

backend/app/api/channels.py

@@ -1,3 +1,5 @@
+import logging
+
 from fastapi import APIRouter, Depends, HTTPException
 from pydantic import BaseModel
 from sqlalchemy import select
@@ -8,6 +10,11 @@ from app.db import get_db
 from app.models.category import Category
 from app.models.channel import Channel
 from app.models.channel_category import channel_categories
+from app.services import sync
+from app.services.google_oauth import OAuthNotConnected
+from app.services.youtube_client import YouTubeAPIError, YouTubeInsufficientScope, YouTubeQuotaExceeded
+
+logger = logging.getLogger(__name__)
 
 router = APIRouter(dependencies=[Depends(require_session)])
 
@@ -104,3 +111,32 @@ def set_channel_categories(channel_id: int, payload: ChannelCategoriesUpdate, db
     db.commit()
 
     return _serialize(channel, sorted(unique_ids))
+
+
+@router.post("/channels/{channel_id}/unsubscribe")
+def unsubscribe_channel(channel_id: int, db: Session = Depends(get_db)) -> dict:
+    channel = db.get(Channel, channel_id)
+    if channel is None:
+        raise HTTPException(status_code=404, detail="Channel not found")
+
+    try:
+        sync.unsubscribe_channel(db, channel)
+    except sync.ChannelHasNoSubscriptionId as exc:
+        raise HTTPException(status_code=400, detail=str(exc))
+    except OAuthNotConnected:
+        raise HTTPException(status_code=400, detail="Google account is not connected")
+    except YouTubeInsufficientScope:
+        raise HTTPException(
+            status_code=403,
+            detail="Недостаточно прав для отписки — переподключи YouTube, чтобы выдать разрешение на управление подписками",
+        )
+    except YouTubeQuotaExceeded:
+        raise HTTPException(status_code=503, detail="YouTube API quota exhausted")
+    except YouTubeAPIError as exc:
+        raise HTTPException(status_code=502, detail=f"YouTube API error: {exc}")
+    except Exception:
+        logger.exception("Failed to unsubscribe from channel %s", channel_id)
+        raise HTTPException(status_code=502, detail="YouTube is unavailable")
+
+    category_map = _category_ids_by_channel(db, [channel_id])
+    return _serialize(channel, category_map.get(channel_id, []))