feat(playlists): two-way (3-way) merge YouTube sync + mandatory preview (S7)

"Sync to YouTube" was a one-directional destructive push (local wins, deletes
from YouTube every item not in the local list), so items added on YouTube were
lost and a dirty pull was skipped entirely. Replace it with a real 3-way merge:
each item is classified against the last-synced baseline as a local-add, a
YouTube-add, or a genuine delete, so an add or delete on either side survives.

- Persist the ordered baseline id set (new playlists.synced_video_ids, migration
  0075); the fingerprint hash alone couldn't drive a merge. NULL baseline (fresh
  export / pre-S7 link) unions losslessly on the first sync and records one.
- Pure merge_plan(baseline, local, youtube) -> add/remove per side + merged order
  (local order, YouTube-only additions appended). merge_playlist applies the
  local side directly and the YouTube side through the failure breaker; the
  baseline only advances on full success so a partial failure re-converges.
- plan_merge/push-plan now report what each side gains and loses; the confirm
  dialog shows the two-way plan before applying (pushPlanMerge, HU+EN).
- Tests: 10 new (pure classification incl. delete-vs-stale-copy; DB apply proving
  a YouTube-add and a Siftlode-add both survive on both sides, a genuine delete
  propagates without collateral loss, no-baseline never deletes, create path).

frontend/src/lib/api/playlists.ts

@@ -31,12 +31,16 @@ export interface PlaylistDetail {
   items: Video[];
 }
 
+// Two-way sync plan: what each side gains/loses under the 3-way merge (neither side wins
+// wholesale). Local edits are free; only the YouTube writes cost quota.
 export interface PushPlan {
   action: "create" | "update";
-  to_insert: number;
-  to_delete: number;
+  add_yt: number; // local-adds to push out to YouTube
+  add_local: number; // YouTube-adds to pull into Siftlode
+  remove_yt: number; // genuine local deletes propagated to YouTube
+  remove_local: number; // genuine YouTube deletes propagated to Siftlode
   to_reorder: number;
-  yt_extra: number; // items on YouTube that the push would remove (divergence)
+  rename: boolean;
   units_estimate: number;
   remaining_today: number;
   affordable: boolean;
@@ -44,9 +48,11 @@ export interface PushPlan {
 
 export interface PushResult {
   created: number;
-  inserted: number;
-  deleted: number;
+  inserted: number; // added on YouTube
+  deleted: number; // removed on YouTube
   reordered: number;
+  pulled: number; // added into Siftlode from YouTube
+  removed_local: number; // removed from Siftlode (deleted on YouTube)
   failures: string[];
   yt_playlist_id: string | null;
 }