[DRAFT] FIX Surface AttackResultEntry.timestamp on hydrated AttackResult

[thirteeneight]

Description

Fixes #1651.

AttackResultEntry persists a non-nullable timestamp column, but get_attack_result() dropped it when rebuilding the domain AttackResult. The backend mapper's attack_result_to_summary then fell back to datetime.now(timezone.utc), so the UI showed today's date on every row no matter when it was actually persisted.

Three small, additive changes:

• Add an optional timestamp: Optional[datetime] = None field to the AttackResult dataclass.
• Pass timestamp=_ensure_utc(self.timestamp) in AttackResultEntry.get_attack_result().
• In attack_result_to_summary, prefer ar.timestamp over datetime.now() when metadata["created_at"] is absent. The metadata override path is preserved unchanged.

Backwards compatible: AttackResultEntries.timestamp has been non-nullable since the entry was introduced, so every existing row has a value to hydrate from. Callers constructing an AttackResult without the new field get None, which the mapper treats the same as missing metadata.

Filed as draft pending maintainer feedback on #1651.

Tests and Documentation

New unit tests:

• tests/unit/models/test_attack_result.py::TestAttackResultTimestamp — default is None, aware datetime is preserved, round-trip through AttackResultEntry, naive SQLite timestamps are normalized to UTC on hydration.
• tests/unit/backend/test_mappers.py::TestAttackResultToSummary — three new cases covering ar.timestamp preferred when metadata is absent, metadata still wins when both are present, fallback to datetime.now() when both are absent.

Verification:

• pytest tests/unit/ — 1339 passed, 0 regressions.
• mypy --strict on the three modified source files — clean.
• ruff format and ruff check — clean.
• No JupyText / docs changes: this is a bug fix and the only public addition is one optional dataclass field.

[thirteeneight]

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