[EPIC] Verify Pendulum DateTime propagation from python-oa3 + README

[dcj]

Summary

Part of the cross-repo ZonedDateTime end-to-end migration. python-oa3-client has zero datetime handling of its own (greps confirm) — it delegates entirely to python-oa3 for body parsing. This issue tracks verification + README time-section.

Work

• After python-oa3 epic lands, run integration tests against a real VTN to confirm pendulum.DateTime propagates through async HTTP / MQTT / webhook channels.
• Add a "Time and timezones" section to README documenting:
  • No datetime handling of its own — delegates to python-oa3
  • All datetime fields surfaced as pendulum.DateTime (zone-aware)
  • Cross-implementation parity with clj-oa3-client after the migration

Cross-references

• Parent epic: https://github.com/grid-coordination/clj-price-server/issues/5
• Design: https://github.com/grid-coordination/clj-price-server/blob/main/doc/time-alternatives.md
• Depends on: [EPIC] Verify Pendulum DateTime parity with clj-oa3 (Option C); README python-oa3#1