feat: automated appointment reminders via email (GRO-23) (#29)

Implements Phase 1 of groombook/groombook#4 — automated email reminders
for upcoming appointments, with booking confirmations sent immediately
on creation.

- **DB**: new `reminder_logs` table tracks sent reminders per appointment
  (unique on appointmentId+type prevents duplicates); `clients` gains
  `email_opt_out` boolean (migration 0004_reminder_logs)
- **Email service**: `apps/api/src/services/email.ts` — nodemailer SMTP
  transport (disabled when SMTP_HOST is unset, so self-hosted installs
  without email config are unaffected); confirmation and reminder email
  templates included
- **Reminder scheduler**: `apps/api/src/services/reminders.ts` — node-cron
  job runs every minute, checks for appointments in the upcoming reminder
  windows (default: 24 h and 2 h), sends emails for opted-in clients,
  and records sends in reminder_logs (idempotent via ON CONFLICT DO NOTHING)
- **Confirmation email**: sent fire-and-forget after successful appointment
  creation (both single and recurring); never blocks the API response
- **Config**: SMTP_HOST, SMTP_PORT, SMTP_SECURE, SMTP_USER, SMTP_PASS,
  SMTP_FROM, REMINDER_HOURS_EARLY, REMINDER_HOURS_LATE env vars documented
  in .env.example; all optional — feature is silently disabled without them
- **Types**: Client.emailOptOut field added to shared types package

Co-authored-by: Groom Book CTO <cto@groombook.app>
Co-authored-by: Paperclip <noreply@paperclip.ing>

.env.example

@@ -14,3 +14,17 @@ OIDC_AUDIENCE=groombook
 # ── API ───────────────────────────────────────────────────────────────────────
 PORT=3000
 CORS_ORIGIN=http://localhost:8080
+
+# ── Email Reminders (optional) ────────────────────────────────────────────────
+# Leave SMTP_HOST unset to disable email notifications entirely.
+# When configured, appointment confirmation and reminder emails are sent via SMTP.
+SMTP_HOST=smtp.example.com
+SMTP_PORT=587
+SMTP_SECURE=false
+SMTP_USER=user@example.com
+SMTP_PASS=password
+SMTP_FROM="Groom Book <noreply@example.com>"
+
+# Hours before appointment to send reminder emails (defaults: 24 and 2)
+REMINDER_HOURS_EARLY=24
+REMINDER_HOURS_LATE=2