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>

prisma/migrations/20260323234807_add_recurring_expenses/migration.sql

@@ -0,0 +1,11 @@
+-- CreateTable
+CREATE TABLE "RecurringExpense" (
+    "id" TEXT NOT NULL PRIMARY KEY,
+    "title" TEXT NOT NULL,
+    "amountCents" INTEGER NOT NULL,
+    "category" TEXT NOT NULL,
+    "dayOfMonth" INTEGER NOT NULL,
+    "active" BOOLEAN NOT NULL DEFAULT true,
+    "createdAt" DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updatedAt" DATETIME NOT NULL
+);

prisma/schema.prisma

@@ -50,6 +50,17 @@ model MerchantCorrection {
   updatedAt    DateTime @updatedAt
 }
 
+model RecurringExpense {
+  id          String   @id @default(cuid())
+  title       String
+  amountCents Int
+  category    Category
+  dayOfMonth  Int // 1–28, capped so it is valid in every month including February
+  active      Boolean  @default(true)
+  createdAt   DateTime @default(now())
+  updatedAt   DateTime @updatedAt
+}
+
 model MonthlyInsight {
   id              String   @id @default(cuid())
   month           String   @unique