feat(gatekeeper): add PR merge automation service

apps/api/prisma/migrations/20260310120000_AddPendingMerges/migration.sql

@@ -0,0 +1,14 @@
+CREATE TABLE "pending_merges" (
+  "id" UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  "repo" TEXT NOT NULL,
+  "pr_number" INTEGER NOT NULL,
+  "head_sha" TEXT NOT NULL,
+  "state" TEXT NOT NULL DEFAULT 'pending',
+  "review_result" JSONB,
+  "ci_status" TEXT,
+  "requester" TEXT,
+  "gitea_merge_url" TEXT,
+  "created_at" TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  "updated_at" TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  CONSTRAINT "pending_merges_repo_pr_sha_key" UNIQUE ("repo", "pr_number", "head_sha")
+);