Add Resend mailer as primary with SMTP fallback for reliable email delivery

src/env.ts

@@ -11,6 +11,8 @@ const schema = z.object({
   DATABASE_CONNECT_TIMEOUT_MS: z.coerce.number().int().positive().default(10_000),
   HOTEL_NAME: z.string().default("Atom"),
   APP_URL: z.string().url().default("http://localhost:3000"),
+  // Resend API key (preferred — simpler HTTP API, always works).
+  RESEND_API_KEY: z.string().optional(),
   // SMTP (password reset / notifications). Email features no-op if unset.
   SMTP_HOST: z.string().optional(),
   SMTP_PORT: z.coerce.number().int().positive().optional(),