docs: add README, LICENSE, .env.example

README in German with universal framing (class, study group, cohort)
rather than INFO1-specific wording. MIT license. Update package.json
with description, keywords, repository URL, proper license field,
start script. Add .env.example documenting required environment
variables.

.env.example

@@ -0,0 +1,11 @@
+# Required
+JWT_SECRET=
+
+# Mail (required for email verification and password reset)
+RESEND_API_KEY=
+MAIL_FROM=noreply@yourdomain.example
+MAIL_FROM_NAME=Class Portal
+
+# App
+APP_URL=https://yourdomain.example
+NODE_ENV=production