refactor(feed): address /code-review high findings on the E2 stack

Review round on the E2 stack (S1-S5). No correctness bugs; the surviving
low/medium findings addressed:

- altitude (#1): extract the shared `tags_by_category` helper used by BOTH
  filtered_query (SQL tag WHERE) and feed_summary (Python facet/count), removing
  the category-grouping duplication, and add bidirectional "keep in lockstep —
  equivalence-tested by test_feed_summary_matches_count_plus_facets" comments so
  the SQL<->Python tag rule can't silently drift.
- test-coverage (#2): the /feed/summary equivalence test seeded only system tags,
  leaving the `ChannelTag.user_id == user.id` visibility branch untested. Add a
  user-owned Tag + link + two combos; the suite is now 346 (was 344) and the new
  cases pass (feed_summary agrees with /feed/count + /facets on user-visible tags).
- dead-code (#3): annotate get_feed_count, get_facets, and the FE api.feedCount/
  facets as "SUPERSEDED by /feed/summary — kept only as the equivalence oracle;
  retire together" so the intentionally-retained surface reads as deliberate.

Not changed (confirmed no-op): the search channel-name resolve (#4) is
once-per-request on a 633-row table and a deliberate pre-resolve for PERF-12's
BitmapOr plan; the facet loop (#5) already computes passes() once per
(channel, category) with a distinct exclude each, so there is no recomputation.

Gate green: ruff + 346 pytest; frontend tsc/knip/prettier (FE change is a comment).

backend/tests/test_feed_views.py

@@ -96,17 +96,22 @@ def _seed_tagged(db, make_user):
         "T1": Tag(user_id=None, name="Tech", category="topic"),
         "T2": Tag(user_id=None, name="Gaming", category="topic"),
         "O1": Tag(user_id=None, name="Favourite", category="other"),
+        # A USER-owned tag (+ a user-owned ChannelTag link below) so the visibility branch
+        # `ChannelTag.user_id == user.id` is exercised, not only the system `user_id IS NULL` one.
+        "U1": Tag(user_id=user.id, name="MyPin", category="other"),
     }
     db.add_all(tags.values())
     chans = {c: Channel(id=c, title=c, subscriber_count=10) for c in ("chA", "chB", "chC", "chD")}
     db.add_all(chans.values())
     db.flush()
-    # channel -> tag keys
+    # channel -> (system) tag keys
     links = {"chA": ["L1", "T1"], "chB": ["L1", "T2"], "chC": ["L2", "T1", "O1"], "chD": []}
     for cid, tkeys in links.items():
         db.add(Subscription(user_id=user.id, channel_id=cid, priority=0, hidden=False))
         for tk in tkeys:
             db.add(ChannelTag(channel_id=cid, tag_id=tags[tk].id, user_id=None))
+    # The user's own private link (visible only to them): chB carries U1 for this user.
+    db.add(ChannelTag(channel_id="chB", tag_id=tags["U1"].id, user_id=user.id))
     # differing video counts per channel (chA:3, chB:2, chC:1, chD:2)
     counts = {"chA": 3, "chB": 2, "chC": 1, "chD": 2}
     for cid, n in counts.items():
@@ -124,6 +129,8 @@ _SUMMARY_CASES = [
     "show=all&tags={L1}&tags={T1}",
     "show=all&tags={T1}&tags={T2}&tag_mode=or",
     "show=all&tags={T1}&tags={T2}&tag_mode=and",
+    "show=all&tags={U1}",  # user-owned tag (visibility == user.id branch)
+    "show=all&tags={U1}&tags={L1}",  # user tag AND a system tag, cross-category
     "show=all&q=clip",
     "show=unwatched",
 ]