feat(channels): surface full-history pull + honest stored/total count on the channel page (S3)

The catalog only stores the recent-backfill/RSS subset of a channel's uploads (video_count is
YouTube's raw total), so a subscribed channel looked like it was "missing" videos. Now the channel
page header shows "{stored} / {total} videos" when there's a gap, and a "Full history" action queues
the deep backfill (the same deep_requested trigger the channel manager uses) — showing a loading
state until done. Backend: channel_detail now returns deep_requested/backfill_done.

frontend/src/i18n/locales/en/channel.json

@@ -10,6 +10,7 @@
   "subscribers": "{{formatted}} subscribers",
   "videoCount_one": "{{count}} video",
   "videoCount_other": "{{count}} videos",
+  "videoCountStored": "{{stored}} / {{total}} videos",
   "totalViews": "{{formatted}} views",
   "joined": "Joined {{date}}",
   "loadingVideos": "Loading this channel's videos…",
@@ -24,5 +25,9 @@
   "language": "Language",
   "topics": "Topics",
   "keywords": "Keywords",
-  "showInFeed": "Show in my feed"
+  "showInFeed": "Show in my feed",
+  "fullHistory": "Full history",
+  "fullHistoryLoading": "Fetching history…",
+  "fullHistoryQueued": "Fetching this channel's full history — new videos will appear as they're found.",
+  "fullHistoryFailed": "Couldn't start the full-history fetch."
 }

frontend/src/i18n/locales/hu/channel.json

@@ -10,6 +10,7 @@
   "subscribers": "{{formatted}} feliratkozó",
   "videoCount_one": "{{count}} videó",
   "videoCount_other": "{{count}} videó",
+  "videoCountStored": "{{stored}} / {{total}} videó",
   "totalViews": "{{formatted}} megtekintés",
   "joined": "Csatlakozott: {{date}}",
   "loadingVideos": "A csatorna videóinak betöltése…",
@@ -24,5 +25,9 @@
   "language": "Nyelv",
   "topics": "Témák",
   "keywords": "Kulcsszavak",
-  "showInFeed": "Mutasd a hírfolyamomban"
+  "showInFeed": "Mutasd a hírfolyamomban",
+  "fullHistory": "Teljes előzmény",
+  "fullHistoryLoading": "Előzmény letöltése…",
+  "fullHistoryQueued": "A csatorna teljes előzményének letöltése elindult — az új videók feltűnnek, ahogy előkerülnek.",
+  "fullHistoryFailed": "Nem sikerült elindítani a teljes előzmény letöltését."
 }