Rename express/ to backend/ and update references

Update paths in sync.sh, master/main.go, and CLAUDE.md to reflect
the directory rename.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/diachron/handlers.ts

@@ -0,0 +1,19 @@
+import { contentTypes } from "./content-types";
+import { core } from "./core";
+import { httpCodes } from "./http-codes";
+import type { Call, Handler, Result } from "./types";
+
+const multiHandler: Handler = async (call: Call): Promise<Result> => {
+    const code = httpCodes.success.OK;
+    const rn = core.random.randomNumber();
+
+    const retval: Result = {
+        code,
+        result: `that was ${call.method} (${rn})`,
+        contentType: contentTypes.text.plain,
+    };
+
+    return retval;
+};
+
+export { multiHandler };