Admin: Get Single User by ID



**Description:**  
Allow admins to retrieve detailed information for a specific user.

**Tasks:**
- [ ] Create `GET /api/admin/users/:id`
- [ ] Require admin role
- [ ] Return full user profile including KYC info (if any)
- [ ] Return `404` if user not found

**Acceptance Criteria:**  
Returns complete user details for valid ID; `404` for invalid ID.                                        

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