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.

backend/app/feedquery.py

@@ -450,7 +450,11 @@ def feed_years(db: Session, user: User, **params) -> list[dict]:
         **{**params, "published_after": None, "published_before": None, "max_age_days": None},
     )
     pub = base.with_only_columns(Video.published_at.label("pub")).subquery()
-    year = func.extract("year", pub.c.pub)
+    # Bucket by UTC year to stay in lockstep with the year filter's boundaries (published_after/before
+    # are pinned to UTC in filtered_query). published_at is timestamptz, so a bare extract('year', …)
+    # would use the DB session timezone and, on a non-UTC session, bucket a video near a year boundary
+    # into a different year than the filter includes — the count and the drill-down feed would disagree.
+    year = func.extract("year", func.timezone("UTC", pub.c.pub))
     rows = db.execute(
         select(year.label("y"), func.count()).group_by(year).order_by(year.desc())
     ).all()