feat: admin article management, sources, settings, rules, and scheduler

- Add article list, editor, and deletion UI.
- Add source management with bias/credibility/active toggles.
- Add encrypted API key CRUD UI.
- Add automated search rules with query, sources, schedule presets,
  and per-rule AI model override.
- Implement node-cron scheduler and instrumentation hook.
- Add admin settings including password change and quota reference.

src/app/api/admin/sources/route.ts

@@ -0,0 +1,10 @@
+import { NextResponse } from 'next/server';
+import { prisma } from '@/lib/db';
+
+export async function GET() {
+  const sources = await prisma.source.findMany({
+    orderBy: { name: 'asc' },
+    include: { _count: { select: { articles: true } } },
+  });
+  return NextResponse.json(sources);
+}