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/urlState.ts

@@ -37,7 +37,7 @@ export function filtersToParams(f: FeedFilters): URLSearchParams {
   if (f.includeLive) p.set("live", "1");
   if (f.tags.length) p.set("tags", f.tags.join(","));
   if (f.tagMode === "and") p.set("tagmode", "and");
-  if (f.channelId) p.set("channel", f.channelId);
+  if (f.channelIds.length) p.set("channel", f.channelIds.join(","));
   if (f.maxAgeDays) p.set("maxage", String(f.maxAgeDays));
   if (f.dateFrom) p.set("from", f.dateFrom);
   if (f.dateTo) p.set("to", f.dateTo);
@@ -72,7 +72,7 @@ export function paramsToFilters(params: URLSearchParams, base: FeedFilters): Fee
       .map((s) => Number(s))
       .filter((n) => Number.isInteger(n));
   if (params.has("tagmode")) f.tagMode = params.get("tagmode") === "and" ? "and" : "or";
-  f.channelId = params.get("channel") || undefined;
+  f.channelIds = (params.get("channel") ?? "").split(",").filter(Boolean);
   f.maxAgeDays = num(params.get("maxage"));
   f.dateFrom = params.get("from") || undefined;
   f.dateTo = params.get("to") || undefined;