feat(feed,channels): "No tag" filter for untagged channels (E8·S1)

Filter to channels you haven't personally tagged in the channel manager
(no user-owned ChannelTag; auto language/topic tags don't count):

- Feed: a "No tag" sentinel in the "Your tags" sidebar combobox (exclusive
  with real tag picks) + a removable "Untagged" chip; new `untagged` bool
  feed param / `notag` URL key, persisted + coerced on every filter path.
- Channel manager: a virtual "No tag" chip in the tag-chip row filters the
  table to untagged channels (client-side, shared NO_TAG_SENTINEL).

Backend `untagged` is a plain channel-level narrowing, so feed_summary keeps
it applied in the base pass — the count/facet lockstep holds with no mirror.

Note: item E (per-tag counts) and item D (Any/All toggle) were already
shipped; S1 delivers only item B.

backend/tests/test_feed_views.py

@@ -162,11 +162,45 @@ _SUMMARY_CASES = [
     "show=all&tags={O1}&tags={O2}&tag_mode_other=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
+    # "No tag" filter (item B): channels with no USER tag. It stays applied in the base pass, so the
+    # facet/count lockstep must still hold with it on (alone, and combined with a positive tag filter).
+    "show=all&untagged=true",
+    "show=all&untagged=true&tags={L1}",
     "show=all&q=clip",
     "show=unwatched",
 ]
 
 
+def test_untagged_filter_keeps_only_channels_the_user_has_not_tagged(db, as_user, make_user):
+    """The "No tag" filter (item B): `untagged=true` keeps only channels with NO tag the USER personally
+    attached (no user-owned ChannelTag). System/auto tags (user_id NULL) don't count, and another user's
+    tag on the same channel doesn't either."""
+    user = make_user(email="untagged@example.com")
+    other = make_user(email="someone-else@example.com")
+    mine = Tag(user_id=user.id, name="Fav", category="other")
+    sys_topic = Tag(user_id=None, name="Tech", category="topic")
+    theirs = Tag(user_id=other.id, name="TheirTag", category="other")
+    db.add_all([mine, sys_topic, theirs])
+    chans = {c: Channel(id=c, title=c, subscriber_count=10) for c in ("chMine", "chSys", "chNone")}
+    db.add_all(chans.values())
+    db.flush()
+    # chMine: the user's own tag → tagged. chSys: only a system tag + another user's tag → still
+    # "untagged" for THIS user. chNone: nothing at all → untagged.
+    db.add(ChannelTag(channel_id="chMine", tag_id=mine.id, user_id=user.id, source="user"))
+    db.add(ChannelTag(channel_id="chSys", tag_id=sys_topic.id, user_id=None, source="auto"))
+    db.add(ChannelTag(channel_id="chSys", tag_id=theirs.id, user_id=other.id, source="user"))
+    for cid in chans:
+        db.add(Subscription(user_id=user.id, channel_id=cid, priority=0, hidden=False))
+        db.add(_video(f"{cid}v", cid, f"{cid} clip"))
+    db.commit()
+    client = as_user(user)
+
+    # Off: every subscribed channel shows.
+    assert _feed_ids(client, "show=all") == {"chMinev", "chSysv", "chNonev"}
+    # On: only channels the user has NOT personally tagged (a system tag / another user's tag don't count).
+    assert _feed_ids(client, "show=all&untagged=true") == {"chSysv", "chNonev"}
+
+
 @pytest.mark.parametrize("case", _SUMMARY_CASES)
 def test_feed_summary_matches_count_plus_facets(db, as_user, make_user, case):
     # /feed/summary must return byte-identical numbers to the old /feed/count + /facets it replaces,