Backend: Implement consistent time handling (UTC-only) and document timestamp sources

[greatest0fallt1me]

Description

Standardize all timestamps to UTC, document which timestamps are derived from chain vs server time, and add tests to prevent accidental local-time usage.

Requirements and context

• Must be tested and documented.
• Must avoid time zone drift bugs in reconciliation and audits.

Suggested execution

git checkout -b chore/utc-time-handling

Implement changes

• Add time helper module and replace ad-hoc Date usage where needed.
• Tests asserting UTC formatting and stable parsing.
• Update docs.

Test and commit

npm test

Example commit message

chore(backend): enforce UTC-only time handling and docs

Guidelines

• Timeframe: 96 hours.