Drug filter WHERE clause used `drug_sequence IS NULL` to keep ancestor nodes,
but levels 0-2 have empty string '' not NULL. Changed to level-based gating:
- Drug filter: `(level < 3 OR drug_sequence LIKE ...)`
- Directorate filter: `(level < 2 OR directory IN (...) OR directory IS NULL OR directory = '')`
- Trust filter was already correct (had `OR trust_name = ''`)
Extract load_data() and load_pathway_data() logic from Reflex AppState
into standalone functions in src/data_processing/pathway_queries.py.
Create thin dash_app/data/queries.py wrapper with DB_PATH resolution.
Andrew Charlwood