feat(finance): Phase 6 — Budget & Analytics

Add monthly budgets per category with spend-vs-budget dashboard and 6-month trend table.
Includes upsert budget API, monthly analytics endpoint, inline budget editing, and route auth fixes.

prisma/schema.prisma

@@ -65,3 +65,15 @@ model transaction_tags {
 
   @@id([transaction_id, tag_id])
 }
+
+model budgets {
+  id           Int      @id @default(autoincrement())
+  owner_id     Int
+  category     String
+  month        DateTime @db.Date
+  amount_limit Decimal  @db.Decimal(10, 2)
+  created_at   DateTime @default(now())
+  updated_at   DateTime @default(now()) @updatedAt
+
+  @@unique([owner_id, category, month])
+}