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;

frontend/src/api/client.ts

@@ -81,6 +81,10 @@ export function setChannelCategories(channelId: number, categoryIds: number[]) {
   })
 }
 
+export function unsubscribeChannel(channelId: number) {
+  return request<ChannelDto>(`/api/channels/${channelId}/unsubscribe`, { method: 'POST' })
+}
+
 export interface CategoryDto {
   id: number
   name: string
@@ -182,6 +186,10 @@ export function downloadVideo(youtubeVideoId: string) {
   return request<DownloadJobDto>(`/api/videos/${youtubeVideoId}/download`, { method: 'POST' })
 }
 
+export function deleteDownload(youtubeVideoId: string) {
+  return request<DownloadJobDto>(`/api/videos/${youtubeVideoId}/download`, { method: 'DELETE' })
+}
+
 export function getDownloadStatus(youtubeVideoId: string) {
   return request<DownloadJobDto>(`/api/videos/${youtubeVideoId}/download-status`)
 }

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) => (

frontend/src/components/DownloadButton.tsx

@@ -1,7 +1,7 @@
 import { useEffect, useRef, useState } from 'react'
 import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query'
 import type { FeedVideoDto } from '../api/client'
-import { downloadVideo, getDownloadStatus } from '../api/client'
+import { deleteDownload, downloadVideo, getDownloadStatus } from '../api/client'
 
 const ACTIVE_STATUSES = ['queued', 'downloading', 'postprocessing']
 
@@ -40,6 +40,13 @@ function DownloadButton({ video }: Props) {
     },
   })
 
+  const deleteMutation = useMutation({
+    mutationFn: () => deleteDownload(video.youtube_video_id),
+    onSuccess: (data) => {
+      queryClient.setQueryData(['download-status', video.youtube_video_id], data)
+    },
+  })
+
   const status = statusQuery.data?.status ?? video.local.status
   const percent = statusQuery.data?.progress_percent ?? video.local.progress_percent
 
@@ -47,7 +54,7 @@ function DownloadButton({ video }: Props) {
   useEffect(() => {
     if (prevStatus.current !== status) {
       prevStatus.current = status
-      if (status === 'completed' || status === 'failed') {
+      if (status === 'completed' || status === 'failed' || status === 'deleted') {
         queryClient.invalidateQueries({ queryKey: ['feed'] })
         queryClient.invalidateQueries({ queryKey: ['video', video.youtube_video_id] })
       }
@@ -55,7 +62,21 @@ function DownloadButton({ video }: Props) {
   }, [status, queryClient, video.youtube_video_id])
 
   if (status === 'completed') {
-    return <span className="download-badge completed">✓ На сервере</span>
+    return (
+      <span className="download-inline-actions">
+        <span className="download-badge completed">✓ На сервере</span>
+        <button
+          onClick={() => {
+            if (confirm('Удалить скачанную копию с сервера?')) {
+              deleteMutation.mutate()
+            }
+          }}
+          disabled={deleteMutation.isPending}
+        >
+          Удалить
+        </button>
+      </span>
+    )
   }
 
   if (status === 'downloading') {