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/tsconfig.json

@@ -0,0 +1,14 @@
+{
+    "compilerOptions": {
+        "esModuleInterop": true,
+        "target": "ES2022",
+        "lib": ["ES2023"],
+        "module": "NodeNext",
+        "moduleResolution": "NodeNext",
+        "noImplicitAny": true,
+        "strict": true,
+        "types": ["node"],
+        "outDir": "out"
+    },
+    "exclude": ["**/*.spec.ts", "**/*.test.ts"]
+}