feat(gatekeeper): add PR merge automation service

apps/api/prisma/schema.prisma

@@ -261,6 +261,23 @@ model User {
   @@map("users")
 }
 
+model PendingMerge {
+  id            String   @id @default(dbgenerated("gen_random_uuid()")) @db.Uuid
+  repo          String
+  prNumber      Int      @map("pr_number")
+  headSha       String   @map("head_sha")
+  state         String   @default("pending")
+  reviewResult  Json?    @map("review_result")
+  ciStatus      String?  @map("ci_status")
+  requester     String?
+  giteaMergeUrl String?  @map("gitea_merge_url")
+  createdAt     DateTime @default(now()) @map("created_at") @db.Timestamptz
+  updatedAt     DateTime @updatedAt @map("updated_at") @db.Timestamptz
+
+  @@unique([repo, prNumber, headSha])
+  @@map("pending_merges")
+}
+
 model UserPreference {
   id        String   @id @default(uuid()) @db.Uuid
   userId    String   @unique @map("user_id") @db.Uuid