All datetimes stored in UTC or with explicit timezone offset

Storing appointment datetimes without timezone information turns a scheduling platform into a time-bomb. A host in PST and a customer in EST reading the same DATETIME 14:00 literal interpret it as different wall-clock times. This violates CWE-686 (wrong type used for value) and ISO 25010 functional correctness. The failure mode is invisible until someone misses an appointment and support has to reconstruct what actually happened. Once daylight saving time transitions enter the picture, the mismatches compound: a DATETIME column that was correct in January silently drifts by an hour in March.

Use TIMESTAMP WITH TIME ZONE (timestamptz) in PostgreSQL for every appointment datetime column, and store a companion customer_timezone VARCHAR for display purposes. Convert incoming local times to UTC at the API boundary using an IANA library.

-- supabase/migrations/YYYYMMDDHHMMSS_appointments.sql
CREATE TABLE appointments (
  id              UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  start_time      TIMESTAMPTZ NOT NULL,
  end_time        TIMESTAMPTZ NOT NULL,
  customer_timezone VARCHAR(50) NOT NULL
);

// src/lib/appointments.ts
import { fromZonedTime } from 'date-fns-tz'

export function toUtc(localIso: string, tz: string): Date {
  return fromZonedTime(new Date(localIso), tz)
}

Call toUtc(body.startTime, body.timezone) before writing to the database in src/app/api/appointments/route.ts.