feat(playlists): YouTube write API client methods

Add OAuth-only write helpers to YouTubeClient (each 50 quota units):
create_playlist, add_playlist_item, move_playlist_item, delete_playlist_item,
delete_playlist, plus iter_playlist_items_with_ids for diffing. A shared _write
helper records quota and raises YouTubeError on non-2xx.

backend/app/youtube/client.py

@@ -213,6 +213,93 @@ class YouTubeClient:
                 f"DELETE subscriptions -> {resp.status_code}: {resp.text[:300]}"
             )
 
+    def iter_playlist_items_with_ids(self, playlist_id: str) -> Iterator[dict]:
+        """Yield {item_id, video_id} for each item of one of the user's playlists, in
+        playlist order. `item_id` is the playlistItem resource id, needed to delete or
+        reposition the item via the write API. OAuth (private playlists work)."""
+        page_token = None
+        while True:
+            params = {
+                "part": "contentDetails",
+                "playlistId": playlist_id,
+                "maxResults": 50,
+            }
+            if page_token:
+                params["pageToken"] = page_token
+            data = self._get("playlistItems", params, cost=1, allow_key=False)
+            for item in data.get("items", []):
+                vid = item.get("contentDetails", {}).get("videoId")
+                if vid:
+                    yield {"item_id": item.get("id"), "video_id": vid}
+            page_token = data.get("nextPageToken")
+            if not page_token:
+                break
+
+    # --- write endpoints (OAuth only, never the API key; each costs 50 quota units) ---
+    def _write(self, method: str, path: str, *, params: dict, json: dict | None = None) -> dict:
+        headers = {"Authorization": f"Bearer {self._access_token()}"}
+        resp = self._http.request(
+            method, f"{API_BASE}/{path}", params=params, json=json, headers=headers
+        )
+        quota.record_usage(self.db, 50)
+        if resp.status_code not in (200, 204):
+            log.warning("YouTube %s %s -> %s: %s", method, path, resp.status_code, resp.text[:200])
+            raise YouTubeError(f"{method} {path} -> {resp.status_code}: {resp.text[:300]}")
+        return resp.json() if resp.content else {}
+
+    def create_playlist(self, title: str, description: str = "", privacy: str = "private") -> str:
+        """Create a new YouTube playlist; returns its id. 50 units."""
+        data = self._write(
+            "POST",
+            "playlists",
+            params={"part": "snippet,status"},
+            json={
+                "snippet": {"title": title[:150], "description": description[:5000]},
+                "status": {"privacyStatus": privacy},
+            },
+        )
+        return data["id"]
+
+    def add_playlist_item(self, playlist_id: str, video_id: str, position: int | None = None) -> str:
+        """Append (or insert at `position`) a video into a playlist; returns the new
+        playlistItem id. 50 units."""
+        snippet = {
+            "playlistId": playlist_id,
+            "resourceId": {"kind": "youtube#video", "videoId": video_id},
+        }
+        if position is not None:
+            snippet["position"] = position
+        data = self._write(
+            "POST", "playlistItems", params={"part": "snippet"}, json={"snippet": snippet}
+        )
+        return data["id"]
+
+    def move_playlist_item(
+        self, item_id: str, playlist_id: str, video_id: str, position: int
+    ) -> None:
+        """Reposition an existing playlistItem to absolute 0-based `position`. 50 units."""
+        self._write(
+            "PUT",
+            "playlistItems",
+            params={"part": "snippet"},
+            json={
+                "id": item_id,
+                "snippet": {
+                    "playlistId": playlist_id,
+                    "position": position,
+                    "resourceId": {"kind": "youtube#video", "videoId": video_id},
+                },
+            },
+        )
+
+    def delete_playlist_item(self, item_id: str) -> None:
+        """Remove a playlistItem from its playlist. 50 units."""
+        self._write("DELETE", "playlistItems", params={"id": item_id})
+
+    def delete_playlist(self, playlist_id: str) -> None:
+        """Delete a whole playlist on YouTube. 50 units."""
+        self._write("DELETE", "playlists", params={"id": playlist_id})
+
     def get_videos(self, video_ids: list[str]) -> list[dict]:
         items: list[dict] = []
         for batch in _chunks(video_ids, 50):