feat: admin usage by endpoint

[Ugooweb]

Closes #613

This PR addresses the backend requirement for the GrantFox FWC26 campaign by implementing the /api/admin/usage/by-endpoint cross-dev usage view. It introduces an endpoint that allows admins to view the top endpoint usage aggregated across all developers.

Changes Made

• New Feature: Added src/routes/admin/usage/by-endpoint.ts to implement the GET /api/admin/usage/by-endpoint route.
• Route Registration: Registered the new admin route in src/app.ts.
• Testing: Added rigorous edge-case and functional testing in src/routes/admin/usage/by-endpoint.test.ts.

Acceptance Criteria Met

• Implementation matches the description (Cross-dev usage view).
• Tests added and passing (focused tests covering edge cases).
• Adhered to repo's lint and code style.
• Secure, efficient, tested, and documented.

Testing & Validation

• Ran the repo's standard test suite (jest).
• Verified database aggregations and SQL clauses correctly filter and limit the grouped endpoints without SQL injection vulnerabilities.

[greatest0fallt1me]

Nice one — clean, focused change. Merging. 🚀