chore(channels): Phase 2 #3 backend cleanup — extract blocked + summary helpers

- Extract _is_blocked(db, user, channel_id): the BlockedChannel EXISTS query was
  copy-pasted in channel_detail, explore_channel, and block_channel.
- Extract _channel_summary(ch): the shared id/title/handle/thumbnail/subscriber/
  video_count projection was hand-rolled in list_channels and discover_channels
  (the detail dict interleaves these with extras, so it's left as-is).

Behavior-neutral. ruff clean, localdev boots healthy.

backend/app/routes/channels.py

@@ -101,12 +101,7 @@ def list_channels(
 
     return [
         {
-            "id": ch.id,
-            "title": ch.title,
-            "handle": ch.handle,
-            "thumbnail_url": ch.thumbnail_url,
-            "subscriber_count": ch.subscriber_count,
-            "video_count": ch.video_count,
+            **_channel_summary(ch),
             "stored_videos": (agg.get(ch.id) or {}).get("stored", 0),
             "last_video_at": (agg.get(ch.id) or {}).get("last_video_at"),
             "total_duration_seconds": (agg.get(ch.id) or {}).get("total_duration_seconds", 0),
@@ -195,12 +190,7 @@ def discover_channels(
 
     return [
         {
-            "id": ch.id,
-            "title": ch.title,
-            "handle": ch.handle,
-            "thumbnail_url": ch.thumbnail_url,
-            "subscriber_count": ch.subscriber_count,
-            "video_count": ch.video_count,
+            **_channel_summary(ch),
             "playlist_video_count": int(vid_count),
             "playlist_count": int(pl_count),
             "details_synced": ch.details_synced_at is not None,
@@ -209,6 +199,18 @@ def discover_channels(
     ]
 
 
+def _channel_summary(ch: Channel) -> dict:
+    """The channel fields common to the list and discovery projections."""
+    return {
+        "id": ch.id,
+        "title": ch.title,
+        "handle": ch.handle,
+        "thumbnail_url": ch.thumbnail_url,
+        "subscriber_count": ch.subscriber_count,
+        "video_count": ch.video_count,
+    }
+
+
 def _channel_detail_dict(
     channel: Channel, *, subscribed: bool, explored: bool, blocked: bool, stored: int
 ) -> dict:
@@ -268,11 +270,7 @@ def channel_detail(
             ExploredChannel.user_id == user.id, ExploredChannel.channel_id == channel_id
         )
     ).first() is not None
-    blocked = db.execute(
-        select(BlockedChannel.id).where(
-            BlockedChannel.user_id == user.id, BlockedChannel.channel_id == channel_id
-        )
-    ).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
     return _channel_detail_dict(
         channel, subscribed=subscribed, explored=explored, blocked=blocked, stored=int(stored)
@@ -295,11 +293,7 @@ def explore_channel(
     channel = db.get(Channel, channel_id)
     if channel is None:
         raise HTTPException(status_code=404, detail="Unknown channel")
-    if db.execute(
-        select(BlockedChannel.id).where(
-            BlockedChannel.user_id == user.id, BlockedChannel.channel_id == channel_id
-        )
-    ).first() is not None:
+    if _is_blocked(db, user, channel_id):
         raise HTTPException(status_code=403, detail="You've blocked this channel.")
     if quota.remaining_today(db) <= sysconfig.get_int(db, "backfill_quota_reserve"):
         raise HTTPException(
@@ -359,6 +353,19 @@ def _user_subscription(db: Session, user: User, channel_id: str) -> Subscription
     return sub
 
 
+def _is_blocked(db: Session, user: User, channel_id: str) -> bool:
+    """Whether this user has blocked this channel."""
+    return (
+        db.execute(
+            select(BlockedChannel.id).where(
+                BlockedChannel.user_id == user.id,
+                BlockedChannel.channel_id == channel_id,
+            )
+        ).first()
+        is not None
+    )
+
+
 @router.patch("/{channel_id}")
 def update_channel(
     channel_id: str,
@@ -546,12 +553,7 @@ def block_channel(
     un-kept search/explore videos are reclaimed by the discovery-cleanup job in due course."""
     if db.get(Channel, channel_id) is None:
         raise HTTPException(status_code=404, detail="Unknown channel")
-    exists = db.execute(
-        select(BlockedChannel.id).where(
-            BlockedChannel.user_id == user.id, BlockedChannel.channel_id == channel_id
-        )
-    ).first()
-    if exists is None:
+    if not _is_blocked(db, user, channel_id):
         db.add(BlockedChannel(user_id=user.id, channel_id=channel_id))
         # Stop any active exploration of it so it can be cleaned up.
         db.execute(