fix(invoices): wrap refund flow in transaction for idempotency safety

- Wrap idempotency check + processRefund() + db.insert() in db.transaction()
- This prevents duplicate Stripe refunds if the DB insert fails after Stripe processes the refund
- Add migration 0027_refunds for the refunds table (was missing)
- Removes out-of-scope changes from PR #278 (csrf.ts, appointmentGroups, appointments, book, groomingLogs, services, stripe-webhooks)

Fixes GRO-637 per CTO review

Co-Authored-By: Paperclip <noreply@paperclip.ing>

packages/db/migrations/0027_refunds.sql

@@ -0,0 +1,11 @@
+CREATE TABLE "refunds" (
+  "id" uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  "invoice_id" uuid NOT NULL REFERENCES "invoices"("id") ON DELETE RESTRICT,
+  "stripe_refund_id" text NOT NULL,
+  "idempotency_key" text UNIQUE,
+  "amount_cents" integer,
+  "created_at" timestamp NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX "idx_refunds_invoice_id" ON "refunds"("invoice_id");
+CREATE INDEX "idx_refunds_idempotency_key" ON "refunds"("idempotency_key");

packages/db/migrations/meta/_journal.json

@@ -190,6 +190,13 @@
       "when": 1775568867192,
       "tag": "0026_stripe_payment",
       "breakpoints": true
+    },
+    {
+      "idx": 27,
+      "version": "7",
+      "when": 1775655267192,
+      "tag": "0027_refunds",
+      "breakpoints": true
     }
   ]
 }