feat(cameo): detect recognised actors not credited in a title

Add two cameo hunters that flag actors recognised in a title but absent from
its cast:
  - cameo_jellyfin.py — pure-Jellyfin cast-membership check (no id cross-walk)
  - cameo_hunt.py     — TMDB filmography check (actor's combined_credits)

run_from_jellyfin.py now stamps the analysed title's Jellyfin item GUID into
the output JSON as top-level 'jellyfin_item_id' (scene_analyze can't know it),
which cameo_jellyfin.py uses to look up the cast in Jellyfin's own id space.
Document that field in the result-sink output schema header.

src/nodes/result_sink_node.hpp

@@ -22,6 +22,11 @@ using json = nlohmann::json;
 //     "schema_version": 1, "movie": "...", "sample_fps": ..., "anneal_sec": ...,
 //     "actors": [{ "name", "imdb_id", "tmdb_id", "jellyfin_id", "scenes": [[t0,t1], ...] }]
 //   }
+//   An optional top-level "jellyfin_item_id" (the analysed title's Jellyfin item
+//   GUID) may also be present: scene_analyze doesn't know it, so it's stamped in
+//   by run_from_jellyfin.py after analysis. Downstream tools (cameo detection)
+//   use it to check cast membership in Jellyfin's own id space — see
+//   scripts/cameo_jellyfin.py.
 //   This is the spec consumed by the Jellyfin plugin: each actor carries every
 //   identity key the gallery knows (empty string if not resolved). The plugin
 //   should prefer "jellyfin_id" (direct Person item GUID) when non-empty, and