feat(api): 回忆录管线简化、路由延迟池与相关加固

- Phase1/2：移除 MemoirOrchestrator.run 与 process_memoir_segments 别名；文档改为 process_memoir_phase1。
- 槽位校验集中到 stage_constants（filter_stage_slots），批处理与顺序路径及 state_service 写库一致。
- StoryRoute：no_llm/parse_error/invalid_target 保守 new_story；短篇护栏不覆盖这些 fallback。
- Phase2 低置信单路径可选延迟（StoryPipelineResult.deferred）：不写 Chapter/Story，Segment 记录 defer 元数据，冷却内不重复消费；上限后停自动重试，Phase1 同类目新段唤醒池内段。
- Alembic 0017：segments 表 narrative_defer_* 列。
- ProfileAgent：经 LlmGateway/注入 Provider 统一聊天与 JSON，新增测试。
- ImagePromptOrchestrator：LLM 初始化失败可依配置降级或硬失败；补充策略测试。
- 配套单测与 README/本地开发文档表述更新。

Co-authored-by: Cursor <cursoragent@cursor.com>

api/tests/test_image_prompt_policy.py

@@ -121,3 +121,44 @@ def test_cover_fallback_disabled_requires_excerpt(monkeypatch):
             chapter_category="family",
             context_excerpt="",
         )
+
+
+def test_image_prompt_orchestrator_provider_failure_uses_fallback(monkeypatch):
+    from app.agents.image_prompt.orchestrator import get_image_prompt_orchestrator
+
+    class BoomGateway:
+        def langchain_llm_for(self, *_a, **_kw):  # noqa: ANN001
+            raise RuntimeError("provider missing")
+
+    monkeypatch.setattr(
+        "app.agents.image_prompt.orchestrator.settings.image_prompt_fallback_disabled",
+        False,
+    )
+    monkeypatch.setattr("app.core.llm_gateway.LlmGateway", lambda: BoomGateway())
+
+    orch = get_image_prompt_orchestrator()
+    out = orch.build_cover_prompt(
+        chapter_title="T",
+        chapter_category="family",
+        context_excerpt="mountain lake",
+    )
+    assert "mountain lake" in out["prompt"].lower()
+
+
+def test_image_prompt_orchestrator_provider_failure_raises_when_disabled(
+    monkeypatch,
+):
+    from app.agents.image_prompt.orchestrator import get_image_prompt_orchestrator
+
+    class BoomGateway:
+        def langchain_llm_for(self, *_a, **_kw):  # noqa: ANN001
+            raise RuntimeError("provider missing")
+
+    monkeypatch.setattr(
+        "app.agents.image_prompt.orchestrator.settings.image_prompt_fallback_disabled",
+        True,
+    )
+    monkeypatch.setattr("app.core.llm_gateway.LlmGateway", lambda: BoomGateway())
+
+    with pytest.raises(RuntimeError, match="provider missing"):
+        get_image_prompt_orchestrator()