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/i18n/locales/en/sidebar.json

@@ -38,7 +38,16 @@
     "language": "Language",
     "topic": "Topic",
     "tags": "Your tags",
-    "channel": "Channel"
+    "channel": "Channel",
+    "year": "Year"
+  },
+  "year": {
+    "more": "More…",
+    "moreLabel": "All years",
+    "none": "No years yet",
+    "sortBy": "Sort",
+    "sortYear": "Year",
+    "sortCount": "Count"
   },
   "show": {
     "unwatched": "Unwatched",

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

@@ -38,7 +38,16 @@
     "language": "Nyelv",
     "topic": "Téma",
     "tags": "Saját címkék",
-    "channel": "Csatorna"
+    "channel": "Csatorna",
+    "year": "Év"
+  },
+  "year": {
+    "more": "Több…",
+    "moreLabel": "Összes év",
+    "none": "Még nincs év",
+    "sortBy": "Rendezés",
+    "sortYear": "Év",
+    "sortCount": "Darab"
   },
   "show": {
     "unwatched": "Nem nézett",