fix: timezone offset aware datetime mixin

[Besthope-Official]

Summary of Changes

use DateTime field and a timezone factory function

Breaking Changes

add a timezone setting config

Checklist

• Issue discussion completed before opening PR
• Scope is small and focused (single feature/fix)
• All functions have full type annotations
• Async/await used for all I/O operations
• Tests added for new behaviors

[Besthope-Official]

Storing timezone-offset timestamps in the database breaks datetime comparisons across different offsets and violates standard practice.

Action needed:

1. Revert TimestampMixin to store UTC only
2. Move timezone conversion to presentation layer (see Add timezone conversion in API serialization layer #124 )

The settings.app.timezone config may still be useful for serialization but should not affect storage.