feat(inbound-api): machine-readable error codes per spec (incl. SITE_UNREACHABLE) + 415 for non-JSON bodies

Claude-Session: https://claude.ai/code/session_01MtdgwpEeCUn6cUA5f1LMPj

src/ZB.MOM.WW.ScadaBridge.InboundAPI/InboundApiEndpointFilter.cs

@@ -56,7 +56,7 @@ public sealed class InboundApiEndpointFilter : IEndpointFilter
             _logger.LogWarning(
                 "Inbound API request rejected — this node is a standby (not the active central node)");
             return Results.Json(
-                new { error = "Inbound API is only available on the active central node" },
+                new { error = "Inbound API is only available on the active central node", code = "STANDBY_NODE" },
                 statusCode: StatusCodes.Status503ServiceUnavailable);
         }
 
@@ -70,7 +70,7 @@ public sealed class InboundApiEndpointFilter : IEndpointFilter
                 "Inbound API request rejected — body length {Length} exceeds limit {Limit}",
                 declaredLength, maxBytes);
             return Results.Json(
-                new { error = "Request body too large" },
+                new { error = "Request body too large", code = "BODY_TOO_LARGE" },
                 statusCode: StatusCodes.Status413PayloadTooLarge);
         }