fix(playlists): dirty lagged one edit (flush before fingerprinting)

The session has autoflush off, so recompute_dirty's query read the last-committed
state, not the current request's pending item/order edits — so the first reorder
after a sync compared against itself and missed dirty, and only the next edit showed
it. Flush pending changes before computing the current fingerprint. Verified: a swap
now flips dirty immediately, and reverting to the original clears it.

backend/app/sync/playlists.py

@@ -43,7 +43,10 @@ def recompute_dirty(db: Session, pl: Playlist) -> None:
     (and Watch later) are never dirty; a missing baseline counts as dirty (unknown YT state)."""
     if not pl.yt_playlist_id:
         pl.dirty = False
-    else:
+        return
+    # The session has autoflush off, so the caller's pending item/order/name edits aren't
+    # visible to current_fingerprint's query yet — flush first or dirty would lag one edit.
+    db.flush()
     pl.dirty = pl.synced_fingerprint != current_fingerprint(db, pl)