tweak(database): replace moment-timezone with date-fns so browser bundles don't ship the timezone database

[jaskfla]

Summary

Part 2 of the DataTrak startup-JS series. Fully independent of the other PRs.

@tupaia/database's browser entry pulls in every model, so moment-timezone (and its ~800 KB timezone database) shipped to every DataTrak user. There were three import paths, all now replaced with the date-fns/date-fns-tz equivalents that are already dependencies:

• SurveyResponse/saveToDatabase.js — momentTimezone(value).tz(tz).format() → the existing formatDateInTimezone(value, tz) helper from @tupaia/utils, whose default format produces byte-identical output (verified below).
• OneTimeLogin.js — moment().subtract(1, 'h').isAfter(...) → isAfter(subHours(new Date(), 1), ...).
• FeedItem.js — moment(...).format('Y-MM-DD HH:mm:ss.SSSZZ') → format(..., 'yyyy-MM-dd HH:mm:ss.SSSxx').
• tsutils/datetime.ts — getTimezoneNameFromTimestamp was itself implemented with momentTimezone.tz.names(); reimplemented with Intl.supportedValuesOf('timeZone') + getTimezoneOffset from date-fns-tz. Same contract (first zone name whose current offset matches the timestamp's offset); the zone list source changes from moment's tzdb to the runtime's ICU data, so the exact name picked can differ in edge cases, but any matching name was always acceptable here.
• utils/addExportDateAndOriginInExcelExportData.js — the last moment-timezone import reachable from the @tupaia/utils barrel; converted to format/formatInTimeZone.

moment-timezone is removed from the database, utils and tsutils dependency lists (plain moment remains, it's still used elsewhere).

This is the one PR in the series that changes server behaviour too — saveToDatabase.js is on the central-server survey response write path.

Output equivalence (old vs new, byte-for-byte)

momentTimezone(ts).tz(tz).format()  ==  formatDateInTimezone(ts, tz)   for +13:00, Z/UTC, +05:30, -04:00
moment ZZ ('+1300')                 ==  date-fns xx ('+1300')
moment 'Do MMM YYYY' ('31st Jul')   ==  date-fns 'do MMM yyyy'
moment 'DD/MM/YY'                   ==  date-fns 'dd/MM/yy'

Measured effect on packages/datatrak-web/dist (raw, pre-gzip)

The 846 KB momentTimezone chunk is gone entirely; the entry chunk is marginally smaller (8,402 → 8,368 KB). About −880 KB.

Test plan

• yarn workspace @tupaia/tsutils test (includes the getTimezoneNameFromTimestamp test) — 17 passed
• yarn workspace @tupaia/utils test — 512 passed
• yarn workspace @tupaia/database test — could not complete locally (dev RDS test database is currently broken: table entity_parent_child_relation missing); needs CI
• central-server survey response tests in CI