Add test coverage for Database.execute and fetch methods

[tmdeveloper007]

Problem

Database.execute, Database.fetchone, and Database.fetchall in backend/secuscan/database.py execute SQL queries and return rows. No direct unit tests cover these methods independently.

Fix

1. Write unit tests for Database.execute, Database.execute_no_commit, Database.fetchone, and Database.fetchall using an in-memory SQLite database.
2. Cover: successful inserts/updates, parameter binding, missing tables (error propagation), and empty result sets.

Acceptance Criteria

• New test file with 10+ tests covering all four methods.
• All tests pass locally.