Refactor: Rename user_stats to user_profile and restructure schema

- Rename user_stats table to user_profile
- Split full_name into first_name and last_name columns
- Remove message_count (focus on profile data only)
- Add is_bot flag to track bot accounts
- Create migration.sql with automatic data migration and schema conversion
- Update MIGRATION.md with rename and migration instructions
- Update all DatabaseManager methods to use new table structure
- Update profile change detection to use separated name fields

.gitignore

@@ -1,5 +1,5 @@
 config.yaml
-
+activity_log.txt
 venv
 .venv
 *.db