Swap the go-chi/cors middleware for rest.CORS (already a dependency),
removing the go-chi/cors module entirely. Behaviour-preserving:
- with AllowedOrigins "*" and credentials enabled, both reflect the request
Origin into Access-Control-Allow-Origin (a literal "*" is invalid with
credentials)
- preflight responses also vary on Access-Control-Request-Method/-Headers, not
just Origin, matching go-chi/cors so caches don't reuse a preflight response
across different requests
Extract the config into corsMiddleware() in middleware.go and add
TestCorsMiddleware covering origin reflection, credentials, preflight
methods/headers/max-age/Vary, and the no-Origin case. go-chi/cors dropped
from go.mod; the chi router stays until the router migration. go test -race,
vet, golangci-lint, govulncheck clean.