Add comprehensive API endpoint documentation

[Copilot]

Documented all 47 REST endpoints across the Hexathon backend API in a new API_ENDPOINTS.md file.

Endpoints Cataloged

Authentication & Public (2 + 2)

• Root, health check
• Register, login with JWT token generation

Resource Management (15)

• Categories: Full CRUD with admin restrictions
• Items: Full CRUD with optional category filtering
• Cart: View, add, remove items, checkout

Team & Event Management (18)

• Teams: CRUD, password regeneration, bulk operations
• Problem Statements: CRUD, per-team generation with 3-attempt limit, confirmation flow
• Submissions: Create/update with Figma/doc URLs, admin deletion

Participant Management (7)

• Full CRUD with check-in status tracking and search

Documentation Structure

• HTTP methods, paths, request schemas with types
• Auth requirements (JWT) and role-based access (Team vs Admin)
• Query parameters and response formats
• Standard error codes and response schemas

Example endpoint format:

### Admin Endpoints (Admin authentication required)
- **POST** `/api/v1/items/` - Create a new item
  - Body: `{"name": string, "photo_url": string, "description": string, "price": int, "category_id": string}`

Built on Fiber (Go) web framework with /api/v1 base path.

Original prompt

what all endpoints are there in this backend?

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