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/app/feedquery.py

@@ -100,6 +100,11 @@ def serialize(row) -> dict:
     }
 
 
+def _tag_visible(user: User):
+    """A ChannelTag row this user may see: a shared system tag (user_id NULL) or their own."""
+    return or_(ChannelTag.user_id.is_(None), ChannelTag.user_id == user.id)
+
+
 def tags_by_category(db: Session, tag_ids: list[int]) -> dict[str, list[int]]:
     """Group tag ids by their category (language/topic/other). The single source for the category split
     shared by the feed's tag WHERE (filtered_query) and the facet/count summary (feed_summary), so the
@@ -118,6 +123,7 @@ def filtered_query(
     *,
     tags: list[int],
     tag_modes: dict[str, str],
+    untagged: bool = False,
     channel_ids: list[str],
     q: str | None,
     min_duration: int | None,
@@ -294,7 +300,7 @@ def filtered_query(
         # selections but still honours the other categories' filters).
         if exclude_tag_category:
             by_category.pop(exclude_tag_category, None)
-        visible = or_(ChannelTag.user_id.is_(None), ChannelTag.user_id == user.id)
+        visible = _tag_visible(user)
         for category, ids in by_category.items():
             if tag_modes.get(category, "or") == "and" and len(ids) > 1:
                 sub = (
@@ -307,6 +313,18 @@ def filtered_query(
                 sub = select(ChannelTag.channel_id).where(ChannelTag.tag_id.in_(ids), visible)
             query = query.where(Video.channel_id.in_(sub))
 
+    # "No tag" filter (item B): keep only channels the USER has not personally tagged in the channel
+    # manager — i.e. channels with no ChannelTag row owned by this user (user_id == me, source "user").
+    # System/auto tags (user_id NULL) don't count: this is "channels I haven't tagged yet". It's a plain
+    # channel-level narrowing (like channel_ids/date), NOT a tag-category selection, so feed_summary
+    # leaves it applied in the base pass and needs no mirror in passes().
+    if untagged:
+        query = query.where(
+            Video.channel_id.notin_(
+                select(ChannelTag.channel_id).where(ChannelTag.user_id == user.id)
+            )
+        )
+
     # Content type: Normal / Shorts / Live·Upcoming as a union of enabled types.
     # Explicit Watched/Saved/Hidden views show every type so nothing goes missing.
     explicit_view = show in ("watched", "hidden")
@@ -355,8 +373,9 @@ def feed_summary(db: Session, user: User, **params) -> dict:
     within, a category in "and" mode requires ALL its selected tags)."""
     tag_modes = params.get("tag_modes", {})
 
-    # ONE pass: per-channel video counts for the base filter, tags stripped so every candidate channel
-    # is present to facet over.
+    # ONE pass: per-channel video counts for the base filter, the positive tag filter stripped so every
+    # candidate channel is present to facet over. The "No tag" (untagged) filter is a plain channel-level
+    # narrowing, so it STAYS applied here (not stripped) and the facets naturally drill down within it.
     base, _rank = filtered_query(db, user, **{**params, "tags": []})
     ch_col = base.with_only_columns(Video.channel_id).subquery()
     channel_counts: dict[str, int] = dict(
@@ -386,7 +405,8 @@ def feed_summary(db: Session, user: User, **params) -> dict:
 
     def passes(ch_tags: set[int], exclude_cat: str | None) -> bool:
         # The Python mirror of filtered_query's SQL tag block (AND across categories, OR within,
-        # topic-AND requires all selected) — keep in lockstep; see the ⚠️ note there.
+        # topic-AND requires all selected) — keep in lockstep; see the ⚠️ note there. (The "No tag"
+        # filter is applied in the base query above, so it needs no mirror here.)
         for cat, sel in selected_by_cat.items():
             if cat == exclude_cat:
                 continue

backend/app/routes/feed.py

@@ -53,6 +53,8 @@ def _feed_params(
     tag_mode_language: str = "or",
     tag_mode_topic: str = "or",
     tag_mode_other: str = "or",
+    # "No tag" filter: keep only channels the user has not personally tagged (no user-owned ChannelTag).
+    untagged: bool = False,
     channel_ids: list[str] = Query(default=[]),
     q: str | None = None,
     min_duration: int | None = None,
@@ -74,6 +76,7 @@ def _feed_params(
             "topic": tag_mode_topic,
             "other": tag_mode_other,
         },
+        "untagged": untagged,
         "channel_ids": channel_ids,
         "q": q,
         "min_duration": min_duration,

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,