API timestamps lack a timezone on SQLite: fresh uploads show "2h ago" #468

Found during QA of #463. This is pre-existing.

Problem

On SQLite, a file uploaded a moment ago shows "2h ago" in the /file-storage list, in a browser at UTC+2.

The API serializes created_at without a timezone offset (for example 2026-10-14T10:40:52.123456). The browser therefore parses it as local time.

Expected

Timestamps from the API carry an offset (Z or +00:00), so the relative time is correct in every timezone.

Notes

• SQLite does not store the timezone, so a timezone-aware column comes back naive. The fix is probably in DTO serialization, by treating a naive datetime as UTC on output, or in the column type's process_result_value.
• Postgres timestamptz is likely fine already. Check the other modules' list endpoints for the same problem: audit log, background tasks and users.