Fix PHP CI legacy and edge gateway tests

- Match self-serve legacy test double invoice signature.
- Wait for the edge gateway integration database before bootstrapping schema.

services/nginx/app/tests/Integration/EdgeGateway/EdgeGatewayBackendIntegrationTest.php

@@ -384,8 +384,7 @@ function edge_gateway_integration_context(): array
             }
         };
 
-        $db = new db($dbConfig);
-        $db->connect();
+        $db = edge_gateway_integration_wait_for_db($dbConfig);
         $GLOBALS['db'] = $db;
 
         $mysqli = $db->conn();
@@ -412,6 +411,25 @@ function edge_gateway_integration_context(): array
     ];
 }
 
+function edge_gateway_integration_wait_for_db(array $dbConfig): db
+{
+    $deadline = microtime(true) + 60;
+    $lastError = null;
+
+    do {
+        try {
+            $db = new db($dbConfig);
+            $db->connect();
+            return $db;
+        } catch (RuntimeException $exception) {
+            $lastError = $exception;
+            usleep(500000);
+        }
+    } while (microtime(true) < $deadline);
+
+    throw $lastError ?? new RuntimeException('Database connection failed before a connection attempt completed.');
+}
+
 /**
  * @return array{host:string,user:string,password:string,database:string,port:int}
  */