feat(channels): tag editing on the channel detail page (R8 S1 Part 2)

Bring the same tag control to the channel page, next to Subscribe:
- backend: GET /api/channels/{id} now returns this user's `tag_ids` for the channel
  (added to _channel_detail_dict + the ChannelDetail interface), matching the manager
  list shape;
- ChannelPage renders the shared ChannelTagsControl, wired to attach/detach/create with
  an optimistic update on the channel-detail cache (reverting on failure) and a manager
  list invalidation so a tag added here shows there too.

Reuses the R8 popover platform + ChannelTagsControl — no new tag UI.

backend/app/routes/channels.py

@@ -230,6 +230,7 @@ def _channel_detail_dict(
     blocked: bool,
     stored: int,
     deep_requested: bool,
+    tag_ids: list[int],
 ) -> dict:
     return {
         "id": channel.id,
@@ -257,6 +258,9 @@ def _channel_detail_dict(
         "backfill_done": channel.backfill_done,
         "from_explore": channel.from_explore,
         "details_synced": channel.details_synced_at is not None,
+        # This user's personal tag links for the channel (same shape as the manager list), so the
+        # channel page can offer the same tag editor next to Subscribe.
+        "tag_ids": tag_ids,
     }
 
 
@@ -297,6 +301,13 @@ def channel_detail(
     ).first() is not None
     blocked = _is_blocked(db, user, channel_id)
     stored = db.scalar(select(func.count(Video.id)).where(Video.channel_id == channel_id)) or 0
+    tag_ids = list(
+        db.scalars(
+            select(ChannelTag.tag_id).where(
+                ChannelTag.user_id == user.id, ChannelTag.channel_id == channel_id
+            )
+        ).all()
+    )
     return _channel_detail_dict(
         channel,
         subscribed=subscribed,
@@ -304,6 +315,7 @@ def channel_detail(
         blocked=blocked,
         stored=int(stored),
         deep_requested=bool(sub.deep_requested) if sub else False,
+        tag_ids=tag_ids,
     )