feat(feed): filter by year — per-year count facet + year control (E8·S2)

Add a Year filter to the feed rail: quick chips for recent years plus a
'More…' popover listing all years with per-year video counts, sortable by
year or count. Selecting a year narrows the feed to that whole calendar
year (a published_after/before range applied in the api layer) and is
mutually exclusive with the Upload-date control.

Backend: new /api/feed/years endpoint + feed_years() — one grouped query
that honours every filter EXCEPT the publish-date narrowing (dropped), so
the year list stays a live drill-down and switching years never zeroes the
others. FE keys the list on the date-stripped filters, so picking a year
re-queries the feed but not the list.

Wires year through FeedFilters, the filterParams/URL (?year) round-trip,
the active-filter chip row, and the panel layout. i18n HU+EN.

Tests: feed_years count query (date narrowing ignored, other filters
honoured); dateFilter year-clearing; urlState year round-trip.

frontend/src/components/Sidebar.tsx

@@ -15,6 +15,7 @@ import { type PanelItem } from "./PanelGroups";
 import { type MediaSourceKind } from "../lib/mediaSource";
 import ChannelPicker from "./ChannelPicker";
 import DateFilterSelect from "./DateFilterSelect";
+import YearFilter from "./YearFilter";
 import Combobox, { type ComboboxOption } from "./Combobox";
 import { useFeedFilters, useFeedFiltersActions } from "./FeedFiltersProvider";
 import { useChannelsActions } from "./ChannelsProvider";
@@ -145,6 +146,7 @@ export default function Sidebar({
 
   const available: Record<string, boolean> = {
     date: true,
+    year: true,
     channel: true,
     language: languages.length > 0,
     topic: topics.length > 0,
@@ -159,6 +161,8 @@ export default function Sidebar({
         return (
           <DateFilterSelect filters={filters} setFilters={setFilters} presets={DATE_PRESETS} />
         );
+      case "year":
+        return <YearFilter filters={filters} setFilters={setFilters} />;
       case "language":
         return (
           <Combobox