Persist timestamps as UTC to preserve instants across UK DST transitions #817

Problem

Timestamp values are currently persisted as local DateTime values without an offset. This loses information during the UK autumn daylight-saving transition, when the local hour from 01:00 to 01:59 occurs twice.

For example:

• 2026-10-25 01:30 +01:00 represents 00:30 UTC
• 2026-10-25 01:30 +00:00 represents 01:30 UTC

A plain local DateTime cannot distinguish those two instants. Downstream APIs cannot reliably reconstruct the original instant and may return an incorrect DateTimeOffset.

Required changes

• Review the timestamp fields written by this repository and identify those currently stored as local DateTime values.
• Change persistence to retain the original instant, preferably by storing UTC.
• Alternatively, persist the local value together with its original UTC offset.
• Add the required database/entity/migration changes.
• Update write/read mappings and any affected contracts or consumers.
• Add tests covering:
  • Normal GMT timestamps.
  • Normal BST timestamps.
  • Both occurrences of the repeated autumn DST hour.
  • The spring DST transition.
• Define how existing historical records are handled, noting that ambiguous historical local times cannot always be reconstructed without source data.

The service-wide reporting policy remains UK Europe/London; estate-specific timezones are out of scope.

This supports the reporting date/time standardisation work in EstateReportingAPI PR #615.