fix(plex): S5a review round 2 — dup-HU guard + tests for dup-guid & guid-keep

- Correctness: _sync_overlay now first-wins on a guid (a `seen` set) so a duplicate HU entry
  sharing a guid can't overwrite the applied Hungarian metadata or double-count `overlaid`.
- Test-coverage: cover the earlier dup-guid fix (both editions of a shared-guid pair get *_hu,
  counted once) and the guid-keep guard (_apply_item preserves a prior guid when the listing
  carries none, still updates on a real one).

Full backend suite 276 passed / 2 skipped.

backend/app/plex/sync.py

@@ -463,11 +463,13 @@ def _sync_overlay(
     for group in by_guid.values():
         for row in group:
             row.title_hu = row.summary_hu = row.genres_hu = None
+    seen: set[str] = set()  # first HU entry for a guid wins; a dup HU item can't overwrite or double-count
     for meta in _paginate(plex, lib.plex_key, item_type):
         g = _canonical_guid(meta)
         targets = by_guid.get(g) if g else None
-        if not targets:
+        if not targets or g in seen:
             continue
+        seen.add(g)
         title_hu = (meta.get("title") or "").strip() or None
         summary_hu = meta.get("summary")
         genres_hu = _tags(meta, "Genre")