fix(plex): round-2 deep-review follow-ups on the review fixes

- collections: the resync-failed-after-successful-write path now returns 422
  (honest "reconcile on next sync" detail), NOT 502. The frontend treats
  502/503/504 as gateway-unreachable and raises a sticky offline banner
  (api/core.ts RETRIABLE_GATEWAY); the app answered, so a 422 action-error
  toast is correct. Removes the false-offline regression the 502 introduced,
  and — since 422 isn't retried — the create double-write vector with it.
- tests: make the mutate-then-resync rollback assertion non-vacuous — the
  resync stub now flushes a partial local change before raising, so the test
  proves db.rollback() actually undoes it; split into a write-failed (422)
  case and a resync-failed (422 + reconcile detail + rollback) case.
- New-collection dup hint is now muted-tone with its own nameMaybeTaken copy
  (per-library uniqueness means the name may be free here) instead of the red
  authoritative nameTaken, which the still-enabled create flow contradicts.
- history: cover the navCounter post-reload collision guard (ensureNavId
  advances the counter past a higher persisted sfScroll id).

backend/tests/test_plex_collections_filter.py

@@ -238,35 +238,45 @@ def test_remove_item_writes_through_to_plex(db, monkeypatch):
         app.dependency_overrides.clear()
 
 
-def test_mutation_failure_returns_422_and_keeps_local_row(db, monkeypatch):
+def test_mutation_failure_returns_422_write_failed(db, monkeypatch):
+    # The Plex mutation itself raised → the write never landed → 422 "Plex write failed", and resync
+    # is never reached (stub.calls holds only the mutating call).
     _lib, user = _seed_editable(db)
     stub = _CollStub(fail_on="rename_collection")
     client = _admin_client(db, user, monkeypatch, stub)
     try:
         r = client.patch("/api/plex/collections/A", json={"title": "Renamed"})
         assert r.status_code == 422, r.text
+        assert "write failed" in r.json()["detail"].lower()
     finally:
         app.dependency_overrides.clear()
-    row = db.execute(select(PlexCollection).where(PlexCollection.rating_key == "A")).scalars().first()
-    assert row.title == "My Coll"  # rolled back — the write never landed
 
 
-def test_resync_failure_after_successful_write_returns_502(db, monkeypatch):
-    # The mutation landed on Plex; only the follow-up refresh READ failed → 502 "sync pending", NOT a
-    # false 422 "write failed". Regression guard for the mutate-then-resync error split.
+def test_resync_failure_after_successful_write_is_422_and_rolls_back(db, monkeypatch):
+    # The mutation landed on Plex; only the follow-up refresh READ failed. We must (a) NOT claim a
+    # write failure and NOT use a 5xx (which the frontend treats as offline) → 422 with a "reconcile
+    # on the next sync" detail, and (b) roll back any partial local upsert the resync had flushed.
     _lib, user = _seed_editable(db)
     stub = _CollStub()
 
     def _resync_boom(db_, plex, lib_, rk):
+        # Simulate resync having flushed a partial local change before its Plex read blew up — the
+        # route's rollback must undo it (this is what makes the rollback assertion non-vacuous).
+        row = db_.execute(select(PlexCollection).where(PlexCollection.rating_key == rk)).scalars().first()
+        row.title = "PARTIAL_SHOULD_ROLL_BACK"
+        db_.flush()
         raise PlexError("read failed")
 
     client = _admin_client(db, user, monkeypatch, stub, resync=_resync_boom)
     try:
         r = client.patch("/api/plex/collections/A", json={"title": "Renamed"})
-        assert r.status_code == 502, r.text
+        assert r.status_code == 422, r.text
+        assert "reconcile" in r.json()["detail"].lower()
         assert "rename_collection" in stub.calls  # the write DID happen
     finally:
         app.dependency_overrides.clear()
+    row = db.execute(select(PlexCollection).where(PlexCollection.rating_key == "A")).scalars().first()
+    assert row.title == "My Coll"  # the partial resync upsert was rolled back
 
 
 def test_show_collection_gets_montage_art_from_shows(db, monkeypatch):