fix(playlists): reorder position collision + dedup remove helper

BUG (PB1): reorder_items only repositioned the items present in the payload,
leaving any omitted item (e.g. one added concurrently in another tab) at its old
position — which then collides with the freshly assigned 0..N-1 range, so
get_playlist's order-by-position returns a nondeterministic/duplicated order.
Now the omitted items get fresh trailing positions (keeping their relative
order), guaranteeing unique contiguous positions.

CLEANUP (PC4): extract _remove_item(db, pl, video_id, mark_dirty=) mirroring the
existing _add_item — remove_watch_later and remove_item duplicated the same
find-by-(playlist,video)/delete/commit block.

Behavior-neutral for the normal full-payload reorder. ruff clean, localdev boots.

backend/app/routes/playlists.py

@@ -74,6 +74,23 @@ def _add_item(db: Session, pl: Playlist, video_id: str, *, mark_dirty: bool) ->
     return True
 
 
+def _remove_item(db: Session, pl: Playlist, video_id: str, *, mark_dirty: bool) -> bool:
+    """Remove a video from a playlist if present (idempotent). Returns whether it was removed.
+    `mark_dirty` recomputes the YouTube-sync dirty flag (skip for Watch later)."""
+    item = db.scalar(
+        select(PlaylistItem).where(
+            PlaylistItem.playlist_id == pl.id, PlaylistItem.video_id == video_id
+        )
+    )
+    if item is None:
+        return False
+    db.delete(item)
+    if mark_dirty:
+        recompute_dirty(db, pl)
+    db.commit()
+    return True
+
+
 def _summary(
     db: Session,
     pl: Playlist,
@@ -239,14 +256,7 @@ def remove_watch_later(
         )
     )
     if pl is not None:
-        item = db.scalar(
-            select(PlaylistItem).where(
-                PlaylistItem.playlist_id == pl.id, PlaylistItem.video_id == video_id
-            )
-        )
-        if item is not None:
-            db.delete(item)
-            db.commit()
+        _remove_item(db, pl, video_id, mark_dirty=False)
     return {"saved": False}
 
 
@@ -452,15 +462,7 @@ def remove_item(
     db: Session = Depends(get_db),
 ) -> dict:
     pl = _own_playlist(db, user, playlist_id)
-    item = db.scalar(
-        select(PlaylistItem).where(
-            PlaylistItem.playlist_id == pl.id, PlaylistItem.video_id == video_id
-        )
-    )
-    if item is not None:
-        db.delete(item)
-        recompute_dirty(db, pl)
-        db.commit()
+    _remove_item(db, pl, video_id, mark_dirty=True)
     return {"playlist_id": pl.id, "video_id": video_id}
 
 
@@ -480,11 +482,20 @@ def reorder_items(
         ).scalars()
     }
     pos = 0
+    seen: set[str] = set()
     for vid in order:
         it = items.get(vid)
         if it is not None:
             it.position = pos
             pos += 1
+            seen.add(vid)
+    # Any items the payload omitted (e.g. added concurrently in another tab) keep their relative
+    # order but get fresh trailing positions, so no two items collide on the same position.
+    for it in sorted(
+        (it for vid, it in items.items() if vid not in seen), key=lambda i: i.position
+    ):
+        it.position = pos
+        pos += 1
     recompute_dirty(db, pl)
     db.commit()
     return {"playlist_id": pl.id, "count": pos}