Add SMTP infrastructure + auth rate limiting

Foundation for the upcoming email-based invite and password-reset flows.

- IEmailSender abstraction with SmtpEmailSender (MailKit 4.16) and a
  LoggingEmailSender fallback used automatically when SMTP is unconfigured
  so local dev works without a real SMTP server.
- Fixed-window rate limits keyed by remote IP: 10 / 15 min on /login,
  5 / hour on /register. Returns 429 with Retry-After. Bypassed in the
  Testing environment so the existing integration suite is unaffected.
- New env vars (SMTP_*, APP_BASE_URL) plumbed through docker-compose
  and documented in .env.example.

.env.example

@@ -2,3 +2,16 @@ POSTGRES_PASSWORD=change-me-strong-password
 JWT_SECRET=change-me-generate-a-random-64-char-string
 FAMILY_CODE=your-family-invite-phrase
 DOMAIN=yeschef.yourdomain.com
+
+# SMTP — required for password reset and email-based invites.
+# Leave SMTP_HOST empty to fall back to a logging sender (dev only;
+# emails are logged instead of delivered).
+SMTP_HOST=
+SMTP_PORT=587
+SMTP_USERNAME=
+SMTP_PASSWORD=
+SMTP_FROM_ADDRESS=no-reply@yourdomain.com
+SMTP_FROM_NAME=YesChef
+
+# Public base URL used to build links in outgoing emails. Defaults to https://${DOMAIN}.
+APP_BASE_URL=https://yeschef.yourdomain.com