feat(filters): filter the feed by several channels at once (OR)

Pick any number of channels; they OR together and every other filter applies
across the lot — "the last week's unwatched from these three".

`channelId`/`channelName` are replaced outright by `channelIds: string[]`, no
legacy reader (standing rule: no back-compat unless asked — a saved view written
against the old field simply loses its channel filter). The wire and the share
URL move with it: repeated `channel_ids` params, `?channel=a,b`. Backend ORs via
`Video.channel_id.in_()`; the count endpoint shares the same params object.

Two things the migration exposed:
- SavedViewsWidget fed DB blobs straight into serializers that now index an
  array — the FeedFilters type says nothing about what an older build wrote, so
  the blob is coerced where it enters. It white-screened the app before this.
- Chips carried channelName to label themselves. Without it they resolve names
  from the channel cache, so they now fall back to the id, not a shared
  "This channel" that would render N identical chips while the cache loads.

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

@@ -8,7 +8,6 @@
   "editHint": "Drag to reorder · eye to show/hide",
   "channel": "Channel",
   "loading": "Loading…",
-  "thisChannel": "This channel",
   "channelCount": "{{count}} channel",
   "channelCount_other": "{{count}} channels",
   "dragToReorder": "Drag to reorder",
@@ -76,5 +75,6 @@
   },
   "channelSearch": "Search channels…",
   "noChannelMatch": "No channel matches.",
-  "channelOrder": "Priority first, then A–Z"
+  "channelOrder": "Priority first, then A–Z",
+  "channelRemove": "Remove channel: {{name}}"
 }

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

@@ -8,7 +8,6 @@
   "editHint": "Húzd az átrendezéshez · szem a megjelenítéshez/elrejtéshez",
   "channel": "Csatorna",
   "loading": "Betöltés…",
-  "thisChannel": "Ez a csatorna",
   "channelCount": "{{count}} csatorna",
   "channelCount_other": "{{count}} csatorna",
   "dragToReorder": "Húzd az átrendezéshez",
@@ -76,5 +75,6 @@
   },
   "channelSearch": "Csatornák keresése…",
   "noChannelMatch": "Nincs találat.",
-  "channelOrder": "Prioritás szerint, majd A–Z"
+  "channelOrder": "Prioritás szerint, majd A–Z",
+  "channelRemove": "Csatorna eltávolítása: {{name}}"
 }