Add Yoco processing fee tracking to payments, settings, and reports

Estimates and snapshots the Yoco card-processing fee (always church-borne)
on every card payment, lets the admin configure in-person/online fee % per
plan in Settings -> Payments, and surfaces the fee alongside gross revenue
across the Finance/Profit reports, Master Orders, Revenue Detailed, and the
full cashup reconciliation flow (close-out screen, Cashup report, and audit
trail) so payout figures match what actually lands in the bank.

backend/prisma/schema.prisma

@@ -221,6 +221,9 @@ model Payment {
   externalId        String?           @unique
   status            String?
   originalPaymentId String?
+  feeAmount         Float?
+  feeRate           Float?
+  feeChannel        String?
   createdAt         DateTime          @default(now())
   user              User              @relation(fields: [userId], references: [id], onDelete: Restrict)
   registration      Registration?     @relation(fields: [registrationId], references: [id], onDelete: SetNull)
@@ -461,6 +464,7 @@ model EventCashupLine {
   cashup         EventCashup               @relation(fields: [cashupId], references: [id], onDelete: Cascade)
   method         String
   expectedAmount Float                     @default(0)
+  feeAmount      Float?
   actualAmount   Float?
   variance       Float?
   notes          String?