Add timezone conversion in API serialization layer

[Besthope-Official]

Use Case

Database stores UTC timestamps, but API responses should show times in user's timezone (e.g., UTC+8 for Asia/Shanghai).

Currently users see UTC times which appear 8 hours behind local time.

Proposed Solution

Implement timezone conversion in Pydantic serializers or middleware:

• Read settings.app.timezone or user preferences
• Convert UTC timestamps to target timezone in response models
• Keep database storage in UTC

Implementation Notes

Use zoneinfo or pytz for timezone conversion.
Example: dt.astimezone(ZoneInfo('Asia/Shanghai'))

Related

Depends on reverting #88's database storage approach.

[Besthope-Official]

currently no datetime fields are returned in API responses. so this issue will be considered a long-term issue