feat: add Safecast radiation monitor layer

[Rezatachi]

Summary

• Add a /api/radiation endpoint backed by Safecast public radiation measurements.
• Normalize Safecast units into nSv/h and include raw values, capture date, status, and source metadata.
• Add the Radiation Monitors layer to the CBRN layer group and expose the new metadata in map popups.

Validation

• git diff --cached --check passed before commit.
• npm run lint was attempted, but the repository currently fails with existing lint issues across many files, mostly @typescript-eslint/no-explicit-any and React hook dependency warnings. The new src/app/api/radiation/route.ts file was not listed among the lint failures.

[vercel]

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[DeerSpotter]

Did the owner die? This is 18 days later

[Rezatachi]

i want to merge lol