Add unix timestamp and ISO date conversion route

[utahkanz-ops]

Summary

• Added a scoped /api/routes-f/unix-date POST route for unix timestamp and ISO date conversion.

Issue

• Closes feat(routes-f): unix timestamp converter #878

Changes

• Supports mode: to_iso and mode: to_unix.
• Supports unit: s and unit: ms, defaulting to seconds when omitted.
• Handles negative timestamps for pre-1970 dates.
• Added route-local helper/types and Jest coverage for seconds, milliseconds, negative timestamps, and round-trips.

Validation

• git diff --check passed.
• npm ci was attempted but timed out after 10 minutes.
• npm ci --ignore-scripts --no-audit --no-fund was attempted but failed with ENOTEMPTY while cleaning node_modules/next/...; npm also reports the local Node v23.1.0 is outside Jest package supported engines (^18.14.0 || ^20.0.0 || ^22.0.0 || >=24.0.0).
• Focused Jest tests were not run because Jest was not installed after the failed dependency install.

Notes

• All task files are scoped under app/api/routes-f/unix-date/ per the issue constraint.
• Re-opened against dev as requested.

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