feat(routes-f): bounding box from center and radius

[davedumto]

Summary

Compute a lat/lng bounding box around a center point given a radius.

Requirements

• POST { lat, lng, radius_km }
• Return { north, south, east, west }
• Account for latitude convergence of longitude
• Validate coordinate ranges
• Tests verify box dimensions at equator and high latitudes

Scope Constraint - READ BEFORE STARTING

All files for this task must live inside app/api/routes-f/. This includes the route handler, helpers/utilities, types, and tests. Do NOT modify, import from, or add files to lib/, utils/, types/, components/, or anywhere else outside app/api/routes-f/. If you need shared logic, duplicate it inside your subfolder. Keeping everything scoped to this folder is intentional - it keeps these tasks independent and mergeable in any order.