Implement admins CRUD endpoints (DEV-64)

[sowmya314]

Jira ticket link

Jira ticket

Implementation description

• Created Admin CRUD endpoints and refactored following the service-layer architecture pattern

Steps to test

1. POST /api/admins
2. GET /api/admins
3. GET /api/admins/{id}
4. PUT /api/admins/{id}
5. DELETE /api/admins/{id}

What should reviewers focus on?

• Verify CRUD operations work correctly
• Ensure API routes correctly call the service layer
• Confirm no database logic remains in the API file

Checklist

Format for branch, commit, and PR title: docs/GIT.md.

• My branch name includes the Jira ticket key
• My PR name is descriptive and in imperative tense
• My PR name includes the Jira ticket key
• My commit messages are descriptive and in imperative tense. My commits are atomic and trivial commits are squashed or fixup'd into non-trivial commits
• My commit messages include the Jira ticket key
• I have run the appropriate linter(s)
• I have requested a review from the PL, as well as other devs who have background knowledge on this PR or who will be building on top of this PR