* refactor: dedupe edge api json response assembly
* refactor: expand jsonResponse helper to all edge functions
Roll out jsonResponse() from _json-response.js to 16 files (14 handlers
+ 2 shared helpers), eliminating 55 instances of the
new Response(JSON.stringify(...)) boilerplate.
Only exception: health.js uses JSON.stringify(body, null, indent) for
pretty-print mode, which is incompatible with the helper signature.
Replaced local jsonResponse/json() definitions in contact.js,
register-interest.js, and cache-purge.js with the shared import.
