feat(autocert): generate unique ACME key paths per CA directory URL

Previously, ACME keys were stored at a single default path regardless of
which CA directory URL was configured. This caused key conflicts when
using multiple different ACME CAs.

Now, the key path is derived from a SHA256 hash of the CA directory URL,
allowing each CA to have its own key file:
- Default CA (Let's Encrypt): certs/acme.key
- Custom CA: certs/acme_<url_hash_16chars>.key

This enables running certificates against multiple ACME providers without
key collision issues.

internal/autocert/paths.go

@@ -1,8 +1,7 @@
 package autocert
 
 const (
-	certBasePath       = "certs/"
-	CertFileDefault    = certBasePath + "cert.crt"
-	KeyFileDefault     = certBasePath + "priv.key"
-	ACMEKeyFileDefault = certBasePath + "acme.key"
+	certBasePath    = "certs/"
+	CertFileDefault = certBasePath + "cert.crt"
+	KeyFileDefault  = certBasePath + "priv.key"
 )