fix(plex): NULL-safe orphan-people prune (review)

Guard the prune's NOT IN against the classic NULL trap: a JSON-null cast array
element would surface as SQL NULL in the referenced set, making name NOT IN
(…, NULL) evaluate NULL for every row → silently prune nothing. Filter NULLs out
of the subquery. (Plex cast-explore S4 review)

backend/app/plex/sync.py

@@ -159,8 +159,14 @@ _REFERENCED_PEOPLE_SQL = """
 def prune_orphan_people(db: Session) -> int:
     """Delete plex_people (headshot cache) rows whose name no longer appears in any movie/show cast or
     directors — orphaned by a library removal. Returns the number pruned. Commits its own work."""
+    # `WHERE ref IS NOT NULL` matters: a JSON-null array element would surface as a SQL NULL in the
+    # referenced set, and `name NOT IN (…, NULL)` is NULL (never TRUE) for every row — the classic
+    # NOT IN trap that would silently prune nothing. Filtering NULLs out keeps the delete correct.
     res = db.execute(
-        text(f"DELETE FROM plex_people WHERE name NOT IN ({_REFERENCED_PEOPLE_SQL})")
+        text(
+            "DELETE FROM plex_people WHERE name NOT IN "
+            f"(SELECT ref FROM ({_REFERENCED_PEOPLE_SQL}) AS refs(ref) WHERE ref IS NOT NULL)"
+        )
     )
     db.commit()
     return res.rowcount or 0