Classifier replacement notes

Changed only app-level classifier integration.

Kept unchanged:
- letter_generation_system_final_v2.py
- tamil_letter_full_pipeline_package.zip
- question flow
- template-based letter generator

Added:
- classifier/letter_type_classifier.joblib
- classifier/classifier_labels.json

app.py now maps new classifier labels to old template letter type keys.
If confidence is below threshold, it returns unclear and asks the user to enter a clearer request.