fix(deploy): prune unused staging images before digest pull

BUG-994: migrate run 2841 failed writing containerd ingest because the staging host had no space left after accumulated exact-SHA images.

frontend/tests/staging-backend-workflows.test.ts

@@ -1008,7 +1008,13 @@ test("normal deployment checks migrations but never applies them", () => {
   for (const workflow of stagingWorkflows) {
     assert.doesNotMatch(workflow, /SERVICE_RUNTIME_PASSWORD|SERVICE_DATABASE_URL/);
   }
+  // 原值: pull api web 前不回收镜像
+  // 新值: image prune --all --force 必须出现在 pull 之前，且不得 volume prune
+  // 原因: BUG-994 / migrate run 2841 在 staging 主机拉 digest 时 containerd 写满
+  assert.match(runner, /image prune --all --force/);
+  assert.doesNotMatch(runner, /volume prune --/);
   assertOrder(runner, [
+    "image prune --all --force",
     "pull api web",
     "up -d --no-build --pull never --wait postgres",
     "--profile migration-check run --rm migration-checker",
@@ -1036,6 +1042,15 @@ test("manual migration uses only PostgreSQL and the digest-pinned migrator", ()
   assert.match(workflow, /^on:\n\s+workflow_dispatch:/m);
   assert.doesNotMatch(workflow, /workflow_run:|\n\s+push:/);
   assert.match(runner, /"\$\{docker_command\[@\]\}" pull "\$WEB_IMAGE"/);
+  // 原值: docker pull "$WEB_IMAGE" 前不回收镜像
+  // 新值: image prune --all --force 必须出现在 pull 之前，且不得 volume prune
+  // 原因: BUG-994 / migrate run 2841 在 staging 主机拉 digest 时 containerd 写满
+  assert.match(runner, /image prune --all --force/);
+  assert.doesNotMatch(runner, /volume prune --/);
+  assertOrder(runner, [
+    "image prune --all --force",
+    '"${docker_command[@]}" pull "$WEB_IMAGE"',
+  ]);
   assert.match(runner, /up -d --no-build --pull never --wait postgres/);
   assert.match(runner, /-f deploy\/docker-compose\.postgres\.yml/);
   assertOrder(runner, [