Standardize report date and timezone handling #595

Problem

The service mixes DateTime.Today, DateTime.Now, and client-provided DateTime values across handlers and reporting queries. This can produce inconsistent results around midnight, daylight-saving transitions, and estates in different timezones.

Relevant areas:

• EstateReportingAPI/Handlers/CalenderHandler.cs
• EstateReportingAPI.BusinessLogic/ReportingManager.cs
• Transaction and settlement request DTOs

Scope

Introduce a documented date/time policy:

• Define whether API timestamps are UTC or offset-aware.
• Define inclusive/exclusive report range semantics.
• Resolve an estate business timezone consistently.
• Replace direct system-clock calls with an injectable clock abstraction.

Acceptance criteria

• All report endpoints follow the same date-range semantics.
• The estate timezone is applied consistently to day-based reports.
• Clock-dependent logic can be tested deterministically.
• Tests cover midnight and daylight-saving boundary cases.