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/components/ChannelCard.tsx

@@ -2,7 +2,7 @@ import { useEffect, useRef, useState } from 'react'
 import { useMutation, useQueryClient } from '@tanstack/react-query'
 import { Link } from 'react-router-dom'
 import type { CategoryDto, ChannelDto } from '../api/client'
-import { setChannelCategories } from '../api/client'
+import { setChannelCategories, unsubscribeChannel } from '../api/client'
 
 interface Props {
   channel: ChannelDto
@@ -44,6 +44,13 @@ function ChannelCard({ channel, categories }: Props) {
     },
   })
 
+  const unsubscribeMutation = useMutation({
+    mutationFn: () => unsubscribeChannel(channel.id),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['channels'] })
+    },
+  })
+
   function toggleCategory(categoryId: number) {
     const current = new Set(channel.category_ids)
     if (current.has(categoryId)) {
@@ -64,6 +71,22 @@ function ChannelCard({ channel, categories }: Props) {
           {channel.title}
         </Link>
         {!channel.subscribed && <span className="badge">отписан</span>}
+        {channel.subscribed && (
+          <button
+            className="unsubscribe-button"
+            onClick={() => {
+              if (confirm(`Отписаться от канала "${channel.title}" на YouTube?`)) {
+                unsubscribeMutation.mutate()
+              }
+            }}
+            disabled={unsubscribeMutation.isPending}
+          >
+            Отписаться
+          </button>
+        )}
+        {unsubscribeMutation.isError && (
+          <p className="error unsubscribe-error">{(unsubscribeMutation.error as Error).message}</p>
+        )}
 
         <div className="channel-categories">
           {assignedNames.map((name) => (