fix: use timezone-aware UTC webhook timestamps

[JoshSalomon]

Summary

• Replace all four deprecated datetime.utcnow() calls in WebhookEvent with timezone-aware datetime.now(UTC) timestamps, including a per-instance creation-time factory.
• Carry the Jira event's received_at through publish_once into the Redis queue message, so the timestamp consumed by the Jira event adapter is UTC-aware. Calls without an explicit timestamp now receive an aware UTC timestamp from publish_once.
• Add regression coverage for event creation and terminal transitions, the Jira route argument, queue serialization, and the queued Jira delivery.
• Document the stricter event-model test command and the Jira queue timestamp behavior.

This PR is limited to Forge's webhook-event timestamps. It does not include the separate forge-poller acknowledgment fix, change live workflow state, or migrate stored data. Caller-supplied timestamps are preserved as before.

Rebase

Rebased onto main at 08f98d3, which includes the maintainer's CI fixes in #345.

Verification

• Focused tests: 88 passed with deprecation warnings treated as errors: pytest tests/unit/models/test_events.py tests/unit/queue tests/unit/api/routes/test_jira_webhook.py tests/unit/orchestrator/event_adapters/test_registry.py -q -W error::DeprecationWarning.
• Ruff lint and formatting checks pass for the changed Python files.
• git diff --check passes.
• The Redis-backed integration assertion was added but was not run locally; CI will run the integration gate.