feat(design): a channel's and a Plex playlist's one action list (C1) — the row's "…", Delete + undo, an action toast's 8 s floor (E28 S3e2)

- Channels: the row's full-history, hide and unsubscribe buttons become its "…" at the row end
  (useChannelActions: open, pin, YouTube, copy name / link, full history or the admin reset,
  hide with an undo toast, unsubscribe behind its danger confirm); the card layout docks the "…"
  beside the name (ChannelLayoutGrid moreKey). The admin reset's confirm now names the reset.
- Plex playlist page: a title, a show group and a season each open the page's one row menu (a
  title's is its usePlexCardActions list plus the removal); no X at rest; removals take an undo
  toast whose Undo puts every row back at its own index in one request (bulk add `positions`);
  j/k walk the visible rows, Delete removes the walked one; rename and delete in the header's "…".
- notify: a toast with an action lives at least 8 s (the owner's call), whatever the toast setting.

backend/app/routes/plex/playlists.py

@@ -3,7 +3,9 @@
 from datetime import datetime, timezone
 
 from fastapi import APIRouter, Depends
-from pydantic import BaseModel
+from typing import Annotated
+
+from pydantic import BaseModel, Field, model_validator
 from sqlalchemy import delete, func, select, update
 from sqlalchemy.orm import Session
 
@@ -285,16 +287,75 @@ def _items_by_rks(db: Session, rks: list) -> list[PlexItem]:
     return out
 
 
+class PlexItemAddBulkIn(PlexItemKeysIn):
+    # Where each key goes back (E28 S3e2, the list page's "Undo" of a removal): an index over the
+    # ORDERED rows, one per key. The two-call shape (append, then send the whole order) leaves the rows
+    # at the end whenever the second call fails, and the order it sends would overwrite anything
+    # another tab changed in between.
+    positions: list[Annotated[int, Field(ge=0)]] | None = None
+
+    @model_validator(mode="after")
+    def _one_position_per_key(self):
+        if self.positions is not None and len(self.positions) != len(self.item_rating_keys):
+            raise ValueError("positions must pair one-to-one with item_rating_keys")
+        return self
+
+
+def _insert_at(db: Session, pid: int, placed: list[tuple[int, PlexItem]]) -> int:
+    """Insert new rows at their indices (ascending, so each index is where the row stood when the
+    rows before it were back), renumber the whole list 0..n-1, and say how many were added. An item
+    already in the list does not move: the add stays idempotent."""
+    rows = list(
+        db.execute(
+            select(PlexPlaylistItem)
+            .where(PlexPlaylistItem.playlist_id == pid)
+            .order_by(PlexPlaylistItem.position, PlexPlaylistItem.id)
+        ).scalars()
+    )
+    present = {r.item_id for r in rows}
+    added = 0
+    for at, it in sorted(placed, key=lambda p: p[0]):
+        if it.id in present:
+            continue
+        row = PlexPlaylistItem(playlist_id=pid, item_id=it.id, position=0)
+        db.add(row)
+        rows.insert(min(at, len(rows)), row)
+        present.add(it.id)
+        added += 1
+    for i, r in enumerate(rows):
+        r.position = i
+    return added
+
+
 @router.post("/playlists/{pid}/items/bulk")
 def playlist_add_bulk(
     pid: int,
-    body: PlexItemKeysIn,
+    body: PlexItemAddBulkIn,
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
     """Append many items at once (a whole season/show). Already-present items are skipped; the input
-    order is preserved for the newly-added ones. Returns how many were added + the new total."""
+    order is preserved for the newly-added ones. With `positions`, each key is put back at its index
+    instead of appended. Returns how many were added + the new total."""
     pl = owned_or_404(db, PlexPlaylist, pid, user, what="playlist")
+    if body.positions is not None:
+        by_rk = {it.rating_key: it for it in _items_by_rks(db, body.item_rating_keys)}
+        placed = [
+            (at, by_rk[str(rk)])
+            for rk, at in zip(body.item_rating_keys, body.positions, strict=True)
+            if str(rk) in by_rk
+        ]
+        added = _insert_at(db, pid, placed)
+        if added:
+            pl.updated_at = datetime.now(timezone.utc)
+        db.commit()
+        total = (
+            db.scalar(
+                select(func.count(PlexPlaylistItem.id)).where(PlexPlaylistItem.playlist_id == pid)
+            )
+            or 0
+        )
+        return {"ok": True, "added": added, "item_count": total}
     items = _items_by_rks(db, body.item_rating_keys)
     existing = set(
         db.execute(