feat: support configurable Cloudflare secret key names

[russell-parks]

Summary

• add camelCase Cloudflare secret key fields (apiTokenKey, apiKeyKey, emailKey) with defaults that preserve current behavior
• keep the existing uppercase fields as legacy aliases and resolve precedence so old manifests continue to work
• update controller secret lookup, CRD schemas, tests, and auth docs/examples for configurable secret data keys

Testing

• GOTOOLCHAIN=go1.24.2 GOCACHE=/private/tmp/cloudflare-operator-go-build GOMODCACHE=/private/tmp/cloudflare-operator-go-mod /opt/homebrew/bin/go test ./api/v1alpha1 ./api/v1alpha2 ./internal/controller -count=1