Add duplicate pattern check when adding budget rules

Client side catches it immediately (case-insensitive match against
existing rules) and shows an inline error. API also rejects with 409
as the authoritative guard.

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

src/app/api/budget-rules/route.ts

@@ -29,6 +29,11 @@ export async function POST(req: Request) {
   const budget = await prisma.budget.findFirst({ where: { id: budgetId, userId: session.user.id } })
   if (!budget) return NextResponse.json({ error: 'Budget not found' }, { status: 404 })
 
+  const existing = await prisma.budgetRule.findFirst({
+    where: { budgetId, pattern: { equals: pattern, mode: 'insensitive' } },
+  })
+  if (existing) return NextResponse.json({ error: 'Duplicate rule pattern' }, { status: 409 })
+
   const rule = await prisma.budgetRule.create({
     data: { userId: session.user.id, budgetId, pattern },
     select: { id: true, budgetId: true, pattern: true },