feat: initialize monorepo structure with Bun and Turborepo

- Add package.json for the root with workspace configuration and scripts
- Create @gruperly/config package with TypeScript configuration
- Establish shared package @gruperly/shared with Zod schemas for validation
- Implement group, student, and payment schemas using Zod
- Set up TypeScript configuration for shared package
- Document stack architecture and technical specifications in stack.md
- Configure Turborepo with build, dev, lint, and typecheck tasks

apps/api/.env.example

@@ -0,0 +1,6 @@
+# PostgreSQL connection string
+DATABASE_URL="postgresql://postgres:postgres@localhost:5432/gruperly?schema=public"
+
+# Better Auth
+BETTER_AUTH_SECRET="replace-with-a-strong-secret"
+BETTER_AUTH_URL="http://localhost:4000"