feat(plex): prune orphaned people after each sync (S4)

The plex_people headshot cache can outlive its subjects — when a title leaves the
Plex library, cast/crew who appear in nothing else are orphaned. prune_orphan_people
deletes rows whose name no longer appears in ANY movie/show cast_names/directors,
run at the end of every plex_sync (best-effort; a jsonb_typeof='array' guard skips
scalar/null JSONB values). The filterable cast_names self-clean with their row;
this cleans the only per-person cache. Verified: an injected orphan is dropped, real
people kept. (Plex cast-explore S4)

backend/app/plex/sync.py

@@ -12,6 +12,7 @@ Cast + intro/credit markers are fetched lazily on the item-detail/player path (P
 import logging
 from datetime import datetime, timezone
 
+from sqlalchemy import text
 from sqlalchemy.orm import Session
 
 from app import sysconfig
@@ -135,10 +136,36 @@ def sync(db: Session) -> dict:
         db.rollback()
         log.warning("Plex sync failed: %s", e)
         return {"error": str(e)}
+    # Prune the headshot cache: a reconcile may have removed titles, orphaning people who no longer
+    # appear in ANY item's cast/directors — drop those rows, mirroring how Plex drops orphaned metadata.
+    try:
+        stats["people_pruned"] = prune_orphan_people(db)
+    except Exception as e:  # best-effort housekeeping — never fail the sync over it
+        log.warning("Plex people prune failed: %s", e)
     log.info("Plex sync done: %s", stats)
     return stats
 
 
+# The person names still referenced by ANY movie/show (cast OR directors). plex_people rows outside
+# this set are orphans left by a library removal.
+_REFERENCED_PEOPLE_SQL = """
+    SELECT jsonb_array_elements_text(cast_names) FROM plex_items WHERE jsonb_typeof(cast_names) = 'array'
+    UNION SELECT jsonb_array_elements_text(directors) FROM plex_items WHERE jsonb_typeof(directors) = 'array'
+    UNION SELECT jsonb_array_elements_text(cast_names) FROM plex_shows WHERE jsonb_typeof(cast_names) = 'array'
+    UNION SELECT jsonb_array_elements_text(directors) FROM plex_shows WHERE jsonb_typeof(directors) = 'array'
+"""
+
+
+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."""
+    res = db.execute(
+        text(f"DELETE FROM plex_people WHERE name NOT IN ({_REFERENCED_PEOPLE_SQL})")
+    )
+    db.commit()
+    return res.rowcount or 0
+
+
 def _upsert_library(db: Session, key: str, title: str, kind: str) -> PlexLibrary:
     lib = db.query(PlexLibrary).filter_by(plex_key=key).first()
     if lib is None: