feat(GRO-597): Stripe payment backend — schema, service, API, webhooks

Consolidates GRO-605, GRO-606, GRO-608 into a single clean PR:
- GRO-605: Stripe SDK integration + payment service
- GRO-606: Payment API endpoints (pay invoice, payment methods, refunds)
- GRO-608: Stripe webhook handler

Migration consolidation:
- Single 0026_stripe_payment.sql migration adds stripeCustomerId to clients
  and stripe_payment_intent_id, stripe_refund_id, payment_failure_reason to invoices
- Removed duplicate 0027_stripe_identifiers.sql

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

packages/db/migrations/meta/0026_snapshot.json

@@ -1,5 +1,5 @@
 {
-  "id": "0027_stripe_identifiers",
+  "id": "0026_stripe_payment",
   "version": "7",
   "dialect": "postgresql",
   "tables": {

packages/db/migrations/meta/_journal.json

@@ -183,6 +183,13 @@
       "when": 1775482467192,
       "tag": "0025_rate_limit",
       "breakpoints": true
+    },
+    {
+      "idx": 26,
+      "version": "7",
+      "when": 1775568867192,
+      "tag": "0026_stripe_payment",
+      "breakpoints": true
     }
   ]
 }