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>

pnpm-lock.yaml

@@ -40,6 +40,9 @@ importers:
       nodemailer:
         specifier: ^6.9.16
         version: 6.10.1
+      stripe:
+        specifier: ^22.0.0
+        version: 22.0.1(@types/node@22.19.15)
       zod:
         specifier: ^4.3.6
         version: 4.3.6
@@ -4124,6 +4127,15 @@ packages:
   strip-literal@3.1.0:
     resolution: {integrity: sha512-8r3mkIM/2+PpjHoOtiAW8Rg3jJLHaV7xPwG+YRGrv6FP0wwk/toTpATxWYOW0BKdWwl82VT2tFYi5DlROa0Mxg==}
 
+  stripe@22.0.1:
+    resolution: {integrity: sha512-Yw764pZ6s8Xu4CtUZdD5uWOkw6gc9xzO9OKylCuj1gMhMDLbyGbDtaPNNSFE4mB6njYSHESYIVbF1iIzUfAl2g==}
+    engines: {node: '>=18'}
+    peerDependencies:
+      '@types/node': '>=18'
+    peerDependenciesMeta:
+      '@types/node':
+        optional: true
+
   strnum@2.2.1:
     resolution: {integrity: sha512-BwRvNd5/QoAtyW1na1y1LsJGQNvRlkde6Q/ipqqEaivoMdV+B1OMOTVdwR+N/cwVUcIt9PYyHmV8HyexCZSupg==}
 
@@ -8774,6 +8786,10 @@ snapshots:
     dependencies:
       js-tokens: 9.0.1
 
+  stripe@22.0.1(@types/node@22.19.15):
+    optionalDependencies:
+      '@types/node': 22.19.15
+
   strnum@2.2.1: {}
 
   supports-color@7.2.0: