Architecture: Type HTTP queue payloads as JSON values - #144

Summary

• Define recursive JSON payload types with top-level null excluded.
• Carry the payload type through HTTP parsing, queue management, and the selected store.
• Add compile-time contract checks and an HTTP roundtrip for nested null values.

Verification

• deno check main.ts tests/*.ts
• deno test --allow-read --allow-write --allow-net --allow-env --allow-run (364 passed)
• npm run quality:production
• git diff --check

Closes #142