Add recurring expenses with active nav tab highlighting

- Add RecurringExpense model to Prisma schema with migration
- Add lib/recurring-expenses.ts: CRUD + virtual projection per month
- Add /recurring-expenses API routes (GET, POST, PATCH, DELETE)
- Merge projected recurring expenses into dashboard totals and expense list
- Add RecurringExpenseManager component to /add-expense page
- Show amber "Recurring" badge on projected items; hide edit/delete for them
- Highlight active nav tab using usePathname() with hover state
- Fix Turbopack/Prisma stub issue by adding serverExternalPackages to next.config.ts
- Clear stale Turbopack stub in Dockerfile before each build

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

next.config.ts

@@ -1,7 +1,7 @@
 import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
-  /* config options here */
+  serverExternalPackages: ["@prisma/client"],
 };
 
 export default nextConfig;