fix(GRO-818): refund button for all paid invoices, inline cardLast4, manual refund for non-Stripe

- Backend refund endpoint: allow refunds on paid invoices without stripePaymentIntentId (manual refund path)
- Backend GET /invoices/🆔 inline fetch cardLast4 + paymentStatus from Stripe when stripePaymentIntentId present
- Frontend: show Refund button on all paid invoices for managers (not just Stripe-backed ones)
- Seed: add stripePaymentIntentId (pi_test_*) to ~20% of paid invoices for Stripe-path testing

cc @cpfarhood

packages/db/src/seed.ts

@@ -978,6 +978,7 @@ async function seed() {
         const invoiceStatus = rand() < 0.95 ? "paid" as const : "pending" as const;
         const paidAt = invoiceStatus === "paid" ? new Date(endTime.getTime() + randInt(5, 30) * 60 * 1000) : null;
 
+        const stripePaymentIntentId = invoiceStatus === "paid" && rand() < 0.2 ? `pi_test_${uuid().replace(/-/g, "").slice(0, 24)}` : null;
         invoiceBatch.push({
           id: invoiceId,
           appointmentId: apptId,
@@ -989,6 +990,7 @@ async function seed() {
           status: invoiceStatus,
           paymentMethod: invoiceStatus === "paid" ? pick(["cash", "card", "card", "card", "check"]) as "cash" | "card" | "check" : null,
           paidAt,
+          stripePaymentIntentId,
           notes: rand() < 0.05 ? "Added extra service at checkout" : null,
         });
 
@@ -1092,13 +1094,14 @@ async function seed() {
       const taxCents = Math.round(effectivePrice * 0.08);
       const totalCents = effectivePrice + taxCents + tipCents;
       const paidAt = new Date(endTime.getTime() + randInt(5, 30) * 60 * 1000);
+      const stripePaymentIntentId = rand() < 0.2 ? `pi_test_${uuid().replace(/-/g, "").slice(0, 24)}` : null;
 
       invoiceBatch.push({
         id: invoiceId, appointmentId: apptId, clientId,
         subtotalCents: effectivePrice, taxCents, tipCents, totalCents,
         status: "paid" as const,
         paymentMethod: pick(["cash", "card", "card", "card", "check"]) as "cash" | "card" | "check",
-        paidAt, notes: null,
+        paidAt, stripePaymentIntentId, notes: null,
       });
       lineItemBatch.push({
         id: uuid(), invoiceId, description: svc.name, quantity: 1,