feat: add Zod schemas, DTOs and typed API route handlers

[Thosine-01]

Summary

Implements typed request/response DTOs and Zod runtime validation for all API routes.

Changes

New files

• src/types/api/index.ts — barrel export
• src/types/api/auth.dto.ts — Login/Signup schemas + DTOs
• src/types/api/courses.dto.ts — CourseListQuery/ById schemas + DTOs
• src/types/api/notes.dto.ts — Notes CRUD schemas + DTOs
• src/types/api/bookmarks.dto.ts — Bookmarks CRUD schemas + DTOs
• src/lib/validation.ts — shared validateBody/validateQuery helpers

Updated files

• src/app/api/courses/route.ts
• src/app/api/courses/[id]/route.ts
• src/app/api/notes/route.ts
• src/app/api/bookmarks/route.ts
• src/app/api/auth/login/route.ts
• src/app/api/auth/signup/route.ts

close #113

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[RUKAYAT-CODER]

Kindly resolve conflict