feat: add forbidden/unauthorized response helpers and usage

[shogun444]

Summary

Adds standardized helpers for forbidden and unauthorized responses and applies them to admin access checks.

Closes #189

Changes

• Enhance sendForbidden and sendUnauthorized to support optional details
• Use sendForbidden in admin controller for consistent 403 handling
• Add unit tests for response helpers

Testing

• pnpm lint
• pnpm build
• pnpm jest src/utils/test/api-response.utils.test.ts

Notes

• Scoped to response helpers and admin access checks only
• No unrelated file changes included

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