Put mount base url in context

src/http/middleware.zig

@@ -465,7 +465,16 @@ pub fn Mount(comptime route: []const u8) type {
             var new_ctx = ctx;
             new_ctx.path = args.path;
 
-            return next.handle(req, res, new_ctx, {});
+            if (@hasField(@TypeOf(new_ctx), "mounted_at")) {
+                new_ctx.mounted_at = try std.fmt.allocPrint(ctx.allocator, "/{s}/{s}", .{
+                    std.mem.trim(u8, ctx.mounted_at, "/"),
+                    std.mem.trim(u8, route, "/"),
+                });
+                defer ctx.allocator.free(new_ctx.mounted_at);
+                return next.handle(req, res, new_ctx, {});
+            } else {
+                return next.handle(req, res, addField(new_ctx, "mounted_at", std.mem.trim(u8, route, "/")), {});
+            }
         }
     };
 }