Hide unsubscribed channels from the channels list, add a Saved tab

- GET /api/channels called with subscribed=true from the Channels page, so
  a channel disappears from the list as soon as it's unsubscribed instead
  of lingering with a badge
- GET /api/feed gains a `downloaded` filter (this was already anticipated
  in the original TZ's feed query params but left unimplemented until
  download_jobs existed) -- matches on each video's *latest* job only, so
  a redownload/delete history doesn't leave stale matches
- New /saved route: same category filtering, video grid and download
  actions as the main feed, scoped to downloaded videos only

2 new backend tests (76 total).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/api/client.ts

@@ -64,12 +64,13 @@ export function getChannel(channelId: number) {
 }
 
 export function listChannels(
-  params: { search?: string; categoryId?: number; uncategorized?: boolean } = {},
+  params: { search?: string; categoryId?: number; uncategorized?: boolean; subscribed?: boolean } = {},
 ) {
   const qs = new URLSearchParams()
   if (params.search) qs.set('search', params.search)
   if (params.categoryId != null) qs.set('category_id', String(params.categoryId))
   if (params.uncategorized) qs.set('uncategorized', 'true')
+  if (params.subscribed != null) qs.set('subscribed', String(params.subscribed))
   const suffix = qs.toString() ? `?${qs.toString()}` : ''
   return request<ChannelDto[]>(`/api/channels${suffix}`)
 }
@@ -199,12 +200,19 @@ export function recheckLocal(youtubeVideoId: string) {
 }
 
 export function getFeed(
-  params: { categoryId?: number; uncategorized?: boolean; channelId?: number; cursor?: string } = {},
+  params: {
+    categoryId?: number
+    uncategorized?: boolean
+    channelId?: number
+    downloaded?: boolean
+    cursor?: string
+  } = {},
 ) {
   const qs = new URLSearchParams()
   if (params.categoryId != null) qs.set('category_id', String(params.categoryId))
   if (params.uncategorized) qs.set('uncategorized', 'true')
   if (params.channelId != null) qs.set('channel_id', String(params.channelId))
+  if (params.downloaded) qs.set('downloaded', 'true')
   if (params.cursor) qs.set('cursor', params.cursor)
   const suffix = qs.toString() ? `?${qs.toString()}` : ''
   return request<{ items: FeedVideoDto[]; next_cursor: string | null }>(`/api/feed${suffix}`)