Channel features: subscribers, new-videos badges, activity-driven sync

- Store subscriber count from YouTube statistics and show it on channel page
- Sync 50 videos per channel with playlistItems pagination support
- Show per-channel and per-category new-videos counters (2-day window)
- Replace hourly videos sync with activity trigger (2h idle) and
  incremental backfill (hard cap 200 per channel)
- Clicking the sidebar new-videos count filters the category feed to
  recent videos only (new_only)
- Update agent-team docs: deploy after green checks

frontend/src/api/client.ts

@@ -58,9 +58,11 @@ export interface ChannelDto {
   description: string | null
   thumbnail_url: string | null
   uploads_playlist_id: string | null
+  subscriber_count: number | null
   subscribed: boolean
   last_synced_at: string | null
   category_ids: number[]
+  new_videos_count: number
 }
 
 export function getChannel(channelId: number) {
@@ -96,6 +98,7 @@ export interface CategoryDto {
   slug: string
   sort_order: number
   channel_count: number
+  new_videos_count: number
 }
 
 export function listCategories() {
@@ -211,6 +214,8 @@ export function getFeed(
     downloaded?: boolean
     search?: string
     cursor?: string
+    limit?: number
+    newOnly?: boolean
   } = {},
 ) {
   const qs = new URLSearchParams()
@@ -219,7 +224,9 @@ export function getFeed(
   if (params.channelId != null) qs.set('channel_id', String(params.channelId))
   if (params.downloaded) qs.set('downloaded', 'true')
   if (params.search) qs.set('search', params.search)
+  if (params.newOnly) qs.set('new_only', 'true')
   if (params.cursor) qs.set('cursor', params.cursor)
+  if (params.limit != null) qs.set('limit', String(params.limit))
   const suffix = qs.toString() ? `?${qs.toString()}` : ''
   return request<{ items: FeedVideoDto[]; next_cursor: string | null }>(`/api/feed${suffix}`)
 }