Add dependency duplication check between app and framework

Adds check-deps.ts which ensures backend/package.json doesn't duplicate
any dependencies already provided by backend/diachron/package.json.
Integrated into backend/check.sh.

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

backend/check-deps.ts

@@ -0,0 +1,66 @@
+import { readFileSync } from "node:fs";
+import { dirname, join } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+
+interface PackageJson {
+    dependencies?: Record<string, string>;
+    devDependencies?: Record<string, string>;
+}
+
+function readPackageJson(path: string): PackageJson {
+    const content = readFileSync(path, "utf-8");
+    return JSON.parse(content);
+}
+
+function getAllDependencyNames(pkg: PackageJson): Set<string> {
+    const names = new Set<string>();
+    for (const name of Object.keys(pkg.dependencies ?? {})) {
+        names.add(name);
+    }
+    for (const name of Object.keys(pkg.devDependencies ?? {})) {
+        names.add(name);
+    }
+    return names;
+}
+
+const diachronPkgPath = join(__dirname, "diachron", "package.json");
+const backendPkgPath = join(__dirname, "package.json");
+
+const diachronPkg = readPackageJson(diachronPkgPath);
+const backendPkg = readPackageJson(backendPkgPath);
+
+const diachronDeps = getAllDependencyNames(diachronPkg);
+const backendDeps = getAllDependencyNames(backendPkg);
+
+const duplicates: string[] = [];
+
+for (const dep of diachronDeps) {
+    if (backendDeps.has(dep)) {
+        duplicates.push(dep);
+    }
+}
+
+if (duplicates.length > 0) {
+    console.error("Error: Duplicate dependencies found.");
+    console.error("");
+    console.error(
+        "The following dependencies exist in both backend/package.json and backend/diachron/package.json:",
+    );
+    console.error("");
+    for (const dep of duplicates.sort()) {
+        console.error(`  - ${dep}`);
+    }
+    console.error("");
+    console.error(
+        "Dependencies in backend/diachron/package.json are provided by the framework",
+    );
+    console.error(
+        "and must not be duplicated in backend/package.json. Remove them from",
+    );
+    console.error("backend/package.json to fix this error.");
+    process.exit(1);
+}
+
+console.log("No duplicate dependencies found.");

backend/check.sh

@@ -11,4 +11,5 @@ out_dir="$check_dir/out"
 source "$check_dir"/../diachron/shims/common
 source "$check_dir"/../diachron/shims/node.common
 
+$ROOT/cmd tsx "$check_dir/check-deps.ts"
 $ROOT/cmd pnpm tsc --outDir "$out_dir"

backend/tsconfig.json

@@ -10,5 +10,5 @@
         "types": ["node"],
         "outDir": "out"
     },
-    "exclude": ["**/*.spec.ts", "**/*.test.ts"]
+    "exclude": ["**/*.spec.ts", "**/*.test.ts", "check-deps.ts"]
 }