Fix account delete blocked by CsvUpload FK constraint

Add onDelete: Cascade to CsvUpload.accountId so deleting an account
cascades to its upload records. Also explicitly delete uploads before
the account in the API route so existing deployed DBs without the
constraint don't error.

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

prisma/schema.prisma

@@ -103,7 +103,7 @@ model BudgetRule {
 model CsvUpload {
   id            String        @id @default(cuid())
   accountId     String
-  account       Account       @relation(fields: [accountId], references: [id])
+  account       Account       @relation(fields: [accountId], references: [id], onDelete: Cascade)
   fileName      String
   rowCount      Int
   importedCount Int