Wires /api/deck-critique to return a structured text-only critique before vision upload support lands.
- accepts deck_text/slides/startup context
- uses NVIDIA JSON mode when available
- falls back to local heuristics when the model is unavailable
- adds focused unittest coverage for validation, fallback, and model JSON paths
Verified with:
- python3 -m unittest tests/test_deck_critique.py -v
- python3 -m py_compile app.py core/*.py tests/test_deck_critique.py
