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/config.ts

@@ -0,0 +1,11 @@
+const config = {
+    database: {
+        user: "abc123",
+        password: "abc123",
+        host: "localhost",
+        port: "5432",
+        database: "abc123",
+    },
+};
+
+export { config };