feat: expose start and completion timestamps for task scan phases

[diksha78dev]

Description

This PR resolves #802 by persisting granular started_at and completed_at timestamps for each scan phase, and exposing them in the get_task_status payload.

Changes Include:

• Schema: Added phase_timestamps_json to the tasks table schema.
• Executor: Initialized the queued timestamp upon task creation.
• Executor: Updated _broadcast_phase to use SQLite's atomic json_set to safely mark the completion of the prior phase and the start of the new phase.
• API: Updated get_task_status to parse and include phase_timestamps in the detail payload.
• Tests: Updated mock task rows and test_broadcast_phase_persists_to_db assertions to handle the expanded JSON timestamp persistence logic.

Related Issues

Closes #802

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Ran unit tests covering task phase broadcasts test_broadcast_phase_persists_to_db to assert that correct SQLite json_set commands are generated.
• Ran unit tests for API integration test_scan_phase_included_in_status_response to verify that phase_timestamps dict is correctly evaluated and serialized in the task status response.

Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.