- upsert_slot: raise 422 immediately if meal_type not in VALID_MEAL_TYPES
- update_prep_task: assert whitelist safety contract after dict comprehension
- CreatePlanRequest: week_start typed as date with must_be_monday validator; str() cast at call site
- PrepTask: frozen=True; build_prep_tasks rewired to use (priority, kwargs) tuples so frozen instances are built with correct sequence_order in one pass (no post-construction mutation)
- Move deferred import json to file-level in meal_plans.py
- Fix test dates: "2026-04-14" was a Tuesday; updated request bodies to "2026-04-13" (Monday)