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/components/SavedViewsWidget.tsx

@@ -26,7 +26,17 @@ import { useConfirm } from "./ConfirmProvider";
 // Compact, order-independent signature of a filter set (reuses the share serializer, which
 // emits only non-default values and ignores the transient shuffle seed). Used to highlight the
 // saved view that matches the feed's current filters.
-const keyOf = (f: FeedFilters) => filtersToParams(f).toString();
+const keyOf = (f: FeedFilters) => filtersToParams(asFilters(f)).toString();
+
+// A saved view's `filters` is a JSON blob straight from the DB — the FeedFilters type says nothing
+// about what an older build actually wrote there, so coerce the array fields before the blob reaches
+// app state or a serializer. Deliberately NOT a migration: a view saved against a dropped field just
+// loses that filter.
+const asFilters = (raw: FeedFilters): FeedFilters => ({
+  ...raw,
+  tags: raw.tags ?? [],
+  channelIds: raw.channelIds ?? [],
+});
 
 function syncDefaultMirror(views: SavedView[]): void {
   const key = accountKey(LS.defaultViewFilters, getActiveAccount());
@@ -99,7 +109,7 @@ export default function SavedViewsWidget({
 
   async function share(v: SavedView) {
     try {
-      await navigator.clipboard.writeText(shareUrl(v.filters));
+      await navigator.clipboard.writeText(shareUrl(asFilters(v.filters)));
       notify({ level: "success", message: t("views.shareCopied") });
     } catch {
       notify({ level: "warning", message: t("sidebar.shareFailed") });
@@ -138,7 +148,7 @@ export default function SavedViewsWidget({
                 view={v}
                 active={keyOf(v.filters) === currentKey}
                 renaming={renameId === v.id}
-                onApply={() => onApply(v.filters)}
+                onApply={() => onApply(asFilters(v.filters))}
                 onStartRename={() => setRenameId(v.id)}
                 onRename={(name) => rename(v.id, name)}
                 onCancelRename={() => setRenameId(null)}