fix(e8): deep-review hardening + cut 0.73.0

Fixes from several multi-agent deep-review passes over the E8 feed-filtering &
channel-manager epic (S1-S4):
- feed_years buckets by UTC year to match the UTC-pinned year-filter bounds
- tag suggestions: examples ∩ subscribed (drop tagged-then-unsubscribed channels
  from the profile denominator); exclude hidden channels as suggestion targets
- channel manager: restore ghost chips + surface an error toast on a failed tag accept
- year filter: debounce the search term so the per-year facet doesn't refetch per keystroke
- channel tags: aria-label the "+N" overflow pills
- extract useTagSuggestions hook; add backend + hook tests (profile boundaries, cap,
  language exclusion, unsubscribed/hidden handling, UTC bucketing, bulk-accept)

Bump VERSION to 0.73.0 + release notes.

frontend/src/i18n/locales/en/channels.json

@@ -52,6 +52,7 @@
     "create": "Create tag",
     "createPlaceholder": "New tag…",
     "createFailed": "Could not create the tag.",
+    "updateFailed": "Could not update the tag.",
     "noTag": "No tag",
     "moreCount_one": "{{count}} more tag",
     "moreCount_other": "{{count}} more tags",

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

@@ -52,6 +52,7 @@
     "create": "Címke létrehozása",
     "createPlaceholder": "Új címke…",
     "createFailed": "A címke létrehozása nem sikerült.",
+    "updateFailed": "A címke frissítése nem sikerült.",
     "noTag": "Nincs címke",
     "moreCount_one": "még {{count}} címke",
     "moreCount_other": "még {{count}} címke",