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).

backend/alembic/versions/0075_playlist_sync_baseline.py

@@ -0,0 +1,26 @@
+"""playlist synced_video_ids (S7: 3-way merge baseline for two-way YouTube sync)
+
+The one-directional "local wins" push is replaced by a 3-way merge that classifies each item
+against the last-synced state. That needs the raw ordered id set as a baseline, not just the
+`synced_fingerprint` hash. Add a nullable `synced_video_ids` (JSON list) column; NULL on every
+existing row means "no baseline yet" — the first merge then unions losslessly and records one.
+Forward-only in practice (a downgrade just drops the column).
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0075_playlist_sync_baseline"
+down_revision: Union[str, None] = "0074_download_credentials"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("playlists", sa.Column("synced_video_ids", sa.Text(), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("playlists", "synced_video_ids")