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/lib/api.ts

@@ -184,8 +184,8 @@ export interface FeedFilters {
   // Library (scope "all") only — provenance filter: "organic" (default) hides search-
   // discovered videos, "all" mixes them in, "search" shows only them. Ignored for "my".
   librarySource?: "organic" | "all" | "search" | "plex";
-  channelId?: string;
-  channelName?: string;
+  /** OR-ed together: a video from ANY of these passes, and every other filter still applies. */
+  channelIds: string[];
   maxAgeDays?: number;
   minDuration?: number;
   maxDuration?: number;
@@ -432,7 +432,7 @@ function filterParams(f: FeedFilters): URLSearchParams {
   p.set("include_shorts", String(f.includeShorts));
   p.set("include_live", String(f.includeLive));
   p.set("show", f.show);
-  if (f.channelId) p.set("channel_id", f.channelId);
+  f.channelIds.forEach((id) => p.append("channel_ids", id));
   if (f.maxAgeDays) p.set("max_age_days", String(f.maxAgeDays));
   if (f.dateFrom) p.set("published_after", f.dateFrom);
   if (f.dateTo) p.set("published_before", f.dateTo);