fix: increase RequeuePeriod from 5s to 15s to honor ZeroSSL's Retry-After

ZeroSSL's ACME server returns 'Retry-After: 15' on order status polls
while in 'processing' state. Certbot (ZeroSSL's reference client via
zerossl-bot) honors this header; cert-manager was polling every 5s,
ignoring it. Changing RequeuePeriod to 15s aligns with ZeroSSL's
expectations and reduces unnecessary API calls.

The correct flow per Certbot's implementation:
  1. POST finalize → 403 orderNotReady (ZeroSSL accepts internally)
  2. Poll → 'processing' + Retry-After: 15
  3. Eventually: poll → 'ready' (ZeroSSL finished internal checks)
  4. POST finalize again → 200 OK + certificate

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: estenrye

pkg/controller/acmeorders/sync.go

@@ -55,9 +55,11 @@ const (
 )
 
 var (
-	// RequeuePeriod is the default period after which an Order should be re-queued.
+	// RequeuePeriod is the default period after which an Order should be re-queued
+	// when in processing state. Set to 15 seconds to match ZeroSSL's Retry-After: 15
+	// response header; this is also a reasonable default for other ACME servers.
 	// It can be overridden in tests.
-	RequeuePeriod = time.Second * 5
+	RequeuePeriod = time.Second * 15
 )
 
 func (c *controller) Sync(ctx context.Context, o *cmacme.Order) (err error) {