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/sync/tagsuggest.py

@@ -51,17 +51,25 @@ def compute_tag_suggestions(db: Session, user: User) -> dict[str, list[dict]]:
     capped at ``MAX_PER_CHANNEL``. Channels with no suggestion are omitted (empty dict when the
     user has no tag with a usable profile). Deterministic for a fixed DB state.
     """
-    # Candidate universe: the channels this user manages. A user can only tag a subscribed channel
-    # (attach_tag requires a subscription), so example channels are a subset of this too — which
-    # lets us bound the auto-tag scan to just these channels.
-    subscribed: set[str] = set(
-        db.scalars(select(Subscription.channel_id).where(Subscription.user_id == user.id)).all()
-    )
+    # Candidate universe: the channels this user is subscribed to. `subscribed` (incl. hidden) is the
+    # SIGNAL set — a hidden channel is still a valid example / topic-feature source, since hiding only
+    # affects the feed, not the tagging pattern. `hidden` is excluded as a suggestion TARGET below (the
+    # Channel Manager shows ghost chips only on non-hidden rows, so suggesting for hidden channels would
+    # inflate the header count past what's shown and let bulk-accept tag channels the user can't see).
+    subs = db.execute(
+        select(Subscription.channel_id, Subscription.hidden).where(Subscription.user_id == user.id)
+    ).all()
+    subscribed: set[str] = {cid for cid, _ in subs}
+    hidden: set[str] = {cid for cid, h in subs if h}
     if not subscribed:
         return {}
 
     # The user's manual tag links: tag_id -> example channels, and each channel's user-tag set (so
-    # a channel with ANY user tag is excluded from "untagged").
+    # a channel with ANY user tag is excluded from "untagged"). Intersect with `subscribed`: unsubscribe
+    # deletes the Subscription but NOT the source='user' ChannelTag, so a tagged-then-unsubscribed
+    # channel would otherwise stay an "example" — inflating a profile's CHAR_SUPPORT denominator while
+    # contributing zero features (features are scanned over `subscribed` only), dropping characteristic
+    # topics. Keeping examples ⊆ subscribed makes the support fraction honest.
     examples: dict[int, set[str]] = {}
     user_tagged: set[str] = set()
     for tag_id, cid in db.execute(
@@ -69,6 +77,8 @@ def compute_tag_suggestions(db: Session, user: User) -> dict[str, list[dict]]:
             ChannelTag.user_id == user.id, ChannelTag.source == "user"
         )
     ).all():
+        if cid not in subscribed:
+            continue
         examples.setdefault(tag_id, set()).add(cid)
         user_tagged.add(cid)
 
@@ -104,7 +114,7 @@ def compute_tag_suggestions(db: Session, user: User) -> dict[str, list[dict]]:
 
     suggestions: dict[str, list[dict]] = {}
     for cid in subscribed:
-        if cid in user_tagged:  # already has a user tag → not "untagged"
+        if cid in hidden or cid in user_tagged:  # hidden → no chips shown; tagged → not "untagged"
             continue
         cfeat = features.get(cid)
         if not cfeat: