fix(api): delete orphaned sessions when character is deleted

- Add delete_sessions_by_character() method to SessionService that
  cleans up all game sessions associated with a character
- Update delete_character() to hard delete instead of soft delete
- Call session cleanup before deleting character to prevent orphaned data
- Delete associated chat messages when cleaning up sessions
- Update API documentation to reflect new behavior

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

api/docs/API_TESTING.md

@@ -634,7 +634,7 @@ curl -X POST http://localhost:5000/api/v1/characters \
 
 **Endpoint:** `DELETE /api/v1/characters/<character_id>`
 
-**Description:** Soft-delete a character (marks as inactive rather than removing).
+**Description:** Permanently delete a character from the database. Also cleans up all associated game sessions to prevent orphaned data.
 
 **Request:**