Use timezone-aware Scan start defaults

[Ghraven]

Summary

• replace the Scan.started_at datetime.utcnow default with a timezone-aware UTC timestamp
• update the matching factory default so tests/builders create the same style of value

Problem

datetime.utcnow() returns a naive datetime object. The rest of this package already uses datetime.now(timezone.utc) in several timestamp paths, so Scan.started_at could be inconsistent with timezone-aware values.

Before / After

Before: newly-created Scan rows and ScanFactory instances received naive UTC datetimes.

After: both defaults produce timezone-aware UTC datetimes.

Verification

• python -m py_compile flowsint-core/src/flowsint_core/core/models.py flowsint-core/tests/factories.py
• $env:AUTH_SECRET='test-secret-for-local-pytest'; $env:REDIS_URL='redis://localhost:6379/0'; $env:MASTER_VAULT_KEY_V1='base64:qnHTmwYb+uoygIw9MsRMY22vS5YPchY+QOi/E79GAvM='; $env:NEO4J_URI_BOLT='bolt://127.0.0.1:7687'; $env:NEO4J_USERNAME='neo4j'; $env:NEO4J_PASSWORD='test-password'; uv run --package flowsint-core pytest flowsint-core/tests/services/test_timezone_timestamps.py flowsint-core/tests/repositories/test_scan_repository.py -q