Fly.io: switch health check method to [[services.http_checks]].

More specifically, remove the dedicated HTTP datapath for health
checks and verify the entire stack, from TLS frontend to S3 backend.
Verifying too little has resulted in a small outage recently when
the pages listener got misconfigured but the health listener happily
accepted connections like normal. This would not happen now that
the health check uses port 443, too.

conf/Caddyfile

@@ -57,11 +57,6 @@
 		protocols h3
 	}
 
-	servers :2002 {
-		name health
-		protocols h1
-	}
-
 	servers :2019 {
 		name metrics
 		protocols h1
@@ -82,9 +77,10 @@
 }
 
 http:// {
-	# initial PUT/POST for a new domain has to happen over HTTP
-	@get method GET
-	redir @get https://{host}{uri} 301
+	# initial PUT/POST for a new domain has to happen over HTTP;
+	# health check also has to reach the backend over HTTP
+	@upgrade `method('GET') && !header({'Health-Check': '*'})`
+	redir @upgrade https://{host}{uri} 301
 
 	import backend
 }
@@ -100,10 +96,6 @@ https:// {
 	import backend
 }
 
-http://localhost:2002 {
-	reverse_proxy http://localhost:3002
-}
-
 http://:2019 {
 	metrics
 }