fix(plex): address S5a review — overlay reconcile, dup-guid, guid-wipe, tests

- Correctness (stale HU): _sync_overlay is now a full reconcile — it clears every guid-bearing EN
  row's *_hu first, then repopulates the matches, so a title whose HU twin vanished or was re-guid'd
  falls back to EN instead of keeping stale Hungarian metadata (only rows that held HU data go dirty).
- Correctness (dup guid): by_guid maps a guid to the LIST of EN rows carrying it, so two editions of
  one film both receive the overlay instead of just the last-iterated one.
- Robustness (guid wipe): _apply_item/_sync_shows keep a prior guid when a listing carries none
  (`_canonical_guid(meta) or row.guid`), so a server ignoring includeGuids can't erase the match key.
- Reuse: merged _sync_overlay's movie/show branches (only the select + item_type differ).
- Test-coverage: a full-sync test drives sync() over a stub Plex (sections + section_items +
  overlay_map) asserting pass-2 designation (metadata_lang/overlay_of), guid capture, the HU write,
  and browse exclusion; plus a reconcile test that a vanished twin clears stale *_hu.

Full backend suite 274 passed / 2 skipped.

backend/app/plex/sync.py

@@ -311,7 +311,9 @@ def _apply_facet_fields(row, meta: dict) -> None:
 def _apply_item(row: PlexItem, lib_id: int, kind: str, meta: dict) -> None:
     row.library_id = lib_id
     row.kind = kind
-    row.guid = _canonical_guid(meta)  # EN<->HU pairing key (movies; harmless/unused on episodes)
+    # EN<->HU pairing key (movies; harmless/unused on episodes). Keep the prior guid if this listing
+    # carries none (a server ignoring includeGuids mustn't wipe the overlay's only match key).
+    row.guid = _canonical_guid(meta) or row.guid
     row.title = (meta.get("title") or "").strip() or "Untitled"
     row.summary = meta.get("summary")
     row.year = meta.get("year")
@@ -366,7 +368,7 @@ def _sync_shows(db: Session, plex: PlexClient, lib: PlexLibrary, stats: dict) ->
             db.add(sh)
             shows[rk] = sh
         sh.library_id = lib.id
-        sh.guid = _canonical_guid(meta)  # EN<->HU pairing key
+        sh.guid = _canonical_guid(meta) or sh.guid  # EN<->HU pairing key; keep prior if listing has none
         sh.title = (meta.get("title") or "").strip() or "Untitled"
         sh.summary = meta.get("summary")
         sh.year = meta.get("year")
@@ -430,39 +432,49 @@ def _sync_overlay(
     db: Session, plex: PlexClient, lib: PlexLibrary, en_lib_id: int, stats: dict
 ) -> None:
     """Write an overlay section's Hungarian metadata onto its EN twin's rows, matched by guid. Creates
-    NO browsable rows — an overlay library is never listed on its own. EN rows with no HU match keep
-    their *_hu NULL (serving falls back to EN); HU items with no EN match are skipped. Only title/
-    summary/genres differ by language; the language-neutral people_text feeds both search vectors."""
+    NO browsable rows — an overlay library is never listed on its own. Only title/summary/genres differ
+    by language; the language-neutral people_text feeds both search vectors.
+
+    This is a full RECONCILE: every guid-bearing EN row's HU fields are cleared first, then the HU
+    section repopulates the matches — so an EN row whose HU twin vanished (or was re-guid'd) falls back
+    to EN instead of keeping stale Hungarian metadata. HU items with no EN match are skipped."""
     if lib.kind == "movie":
-        by_guid = {
-            r.guid: r
-            for r in db.execute(
-                select(PlexItem).where(
-                    PlexItem.library_id == en_lib_id,
-                    PlexItem.kind == "movie",
-                    PlexItem.guid.is_not(None),
-                )
-            ).scalars()
-        }
+        rows = db.execute(
+            select(PlexItem).where(
+                PlexItem.library_id == en_lib_id,
+                PlexItem.kind == "movie",
+                PlexItem.guid.is_not(None),
+            )
+        ).scalars()
         item_type = _TYPE_MOVIE
     else:
-        by_guid = {
-            r.guid: r
-            for r in db.execute(
-                select(PlexShow).where(
-                    PlexShow.library_id == en_lib_id, PlexShow.guid.is_not(None)
-                )
-            ).scalars()
-        }
+        rows = db.execute(
+            select(PlexShow).where(
+                PlexShow.library_id == en_lib_id, PlexShow.guid.is_not(None)
+            )
+        ).scalars()
         item_type = _TYPE_SHOW
+    # guid -> the EN rows carrying it (a list: two editions of one film can share a guid; both overlay).
+    by_guid: dict[str, list] = {}
+    for r in rows:
+        by_guid.setdefault(r.guid, []).append(r)
+    # Reconcile step: clear HU fields on all guid-bearing rows. Only rows that actually held HU data
+    # become dirty (SQLAlchemy skips a no-net-change UPDATE), so this is cheap; matches are re-set below.
+    for group in by_guid.values():
+        for row in group:
+            row.title_hu = row.summary_hu = row.genres_hu = None
     for meta in _paginate(plex, lib.plex_key, item_type):
         g = _canonical_guid(meta)
-        row = by_guid.get(g) if g else None
-        if row is None:
+        targets = by_guid.get(g) if g else None
+        if not targets:
             continue
-        row.title_hu = (meta.get("title") or "").strip() or None
-        row.summary_hu = meta.get("summary")
-        row.genres_hu = _tags(meta, "Genre")
+        title_hu = (meta.get("title") or "").strip() or None
+        summary_hu = meta.get("summary")
+        genres_hu = _tags(meta, "Genre")
+        for row in targets:
+            row.title_hu = title_hu
+            row.summary_hu = summary_hu
+            row.genres_hu = genres_hu
         stats["overlaid"] = stats.get("overlaid", 0) + 1
     db.flush()