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

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: Paperclip <noreply@paperclip.ing>

apps/api/package.json

@@ -18,11 +18,15 @@
     "@hono/zod-validator": "^0.4.3",
     "hono": "^4.6.17",
     "jose": "^5.9.6",
+    "node-cron": "^3.0.3",
+    "nodemailer": "^6.9.16",
     "openid-client": "^6.1.7",
     "zod": "^3.24.1"
   },
   "devDependencies": {
     "@types/node": "^22.10.7",
+    "@types/node-cron": "^3.0.11",
+    "@types/nodemailer": "^6.4.17",
     "eslint": "^9.18.0",
     "tsx": "^4.19.2",
     "typescript": "^5.7.3",