feat(ST-1001): add LoginPairing model + pg_notify trigger via migration

Schema (prisma/schema.prisma):
- model LoginPairing met id (cuid), secret_hash + desktop_token_hash (beide NOT
  NULL — scheiden mobiel- en desktop-bewijs), status (pending|approved|consumed
  |cancelled), optionele user_id met onDelete: SetNull, desktop_ua VarChar(255),
  desktop_ip VarChar(45) voor IPv6, created_at + expires_at + approved_at +
  consumed_at, indexes op (expires_at) en (status, expires_at)
- back-relation login_pairings LoginPairing[] op User

Migratie (20260427200734_add_login_pairing):
- Prisma-gegenereerde DDL voor login_pairings + indexes + FK
- Toegevoegde notify_pairing_change() functie + login_pairings_notify trigger
  op AFTER INSERT/UPDATE; emit pg_notify('scrum4me_pairing', payload) met
  { op: 'I'|'U', pairing_id, status }
- DELETE niet ondersteund — pairings gaan naar consumed/cancelled, niet weg
- Channel naam analoog aan scrum4me_changes uit ST-801

Verification: Node pg-client roundtrip-test via DATABASE_URL toonde notifies bij
INSERT (op=I) en UPDATE (op=U) met correcte payload-shape.

Bouwt voort op M8 LISTEN/NOTIFY-infra. SSE-route /api/auth/pair/stream/[id] in
ST-1004 abonneert hierop.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

prisma/schema.prisma

@@ -65,6 +65,7 @@ model User {
   todos             Todo[]
   product_members   ProductMember[]
   assigned_stories  Story[]         @relation("StoryAssignee")
+  login_pairings    LoginPairing[]
 
   @@index([active_product_id])
   @@map("users")
@@ -247,3 +248,22 @@ model Todo {
   @@index([user_id, product_id])
   @@map("todos")
 }
+
+model LoginPairing {
+  id                 String    @id @default(cuid())
+  secret_hash        String
+  desktop_token_hash String
+  status             String
+  user_id            String?
+  user               User?     @relation(fields: [user_id], references: [id], onDelete: SetNull)
+  desktop_ua         String?   @db.VarChar(255)
+  desktop_ip         String?   @db.VarChar(45)
+  created_at         DateTime  @default(now())
+  expires_at         DateTime
+  approved_at        DateTime?
+  consumed_at        DateTime?
+
+  @@index([expires_at])
+  @@index([status, expires_at])
+  @@map("login_pairings")
+}