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/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,