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>

diachron/common.d/check-deps

@@ -0,0 +1,11 @@
+#!/bin/bash
+
+set -eu
+
+DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+ROOT="$DIR/../.."
+
+cd "$ROOT/backend"
+
+"$ROOT/cmd" tsx check-deps.ts "$@"

diachron/develop.d/check-deps

@@ -0,0 +1 @@
+../common.d/check-deps

diachron/mgmt.d/check-deps

@@ -0,0 +1 @@
+../common.d/check-deps