Remove prompts API and DB migration files

Delete the deprecated prompts feature and its database migration utilities. Removed endpoints and schema definitions (app/api/prompts/__init__.py, prompts.py, schemas.py) along with related utils and SQL/migration scripts (create_prompts_table.*, migrate_prompts_table.py, alter_prompts_table_add_started_completed.sql). Cleans up unused API surface and associated DB artifacts.

Author: goldlabelapps