refactor(agents): 抽取阶段常量与对话上下文；快档 LLM；图片 prompt 可禁止回退

访谈与阶段
- 新增 app/agents/stage_constants.py：集中 CHAT_STAGES、章节分类/顺序、阶段到默认 memoir 类别等，与 MemoirState 默认槽位顺序对齐；减少散落在 prompts 内的重复常量。
- 新增 app/agents/chat/prompt_context.py：以 ChatPromptContext 汇总 guided 系统提示所需字段（阶段、槽位、轮次、人设、记忆证据、回复长度模式、背景声线、职业等），统一走 get_guided_conversation_prompt。
- 大幅收敛 app/agents/chat/prompts_conversation.py；调整 prompts.py、stage_prompts.py、stage_detection.py；同步 interview_agent、profile_agent、helpers 与 state_schema，使对话侧构造提示的方式一致、可测。

回忆录流水线
- memoir/prompts.py 删除已迁至 stage_constants / 独立模板的大段常量与图片占位相关逻辑；classification / extraction / fidelity / narrative agents 与 orchest（全量历史仍可用于计数，注入模型时按轮次与字符上限截断）、image_prompt_fallback_disabled。
- dependencies 增加 get_llm_provider_fast（LRU 缓存，可与默认共用密钥与 base_url）。

任务与编排
- memoir_tasks：prepare_batches 注入 llm_fast；开启独立快档模型时打结构化日志。
- chapter_cover_tasks、story_image_tasks：与图片 prompt / JSON 工具路径或策略变更对齐（import 与行为一致）。
- story_pipeline_sync 等小处同步。

其它核心
- langchain_llm、text_normalize 随上述调用链微调。

开发者体验
- .cursor/settings.json：启用 redis-development、postman 插件。

测试
- 新增 test_image_prompt_policy：覆盖「禁止回退」等图片 prompt 策略。
- 更新 test_interview_prompts、test_interview_reply_length、test_experience_regressions、test_json_and_memory_utils，匹配新常量位置、json_utils 与对话/长度行为。

api/tests/test_image_prompt_policy.py

@@ -0,0 +1,123 @@
+"""MemoirImagePromptService 与 image_prompt_fallback_disabled（story 主图 / 章节封面）。"""
+
+import json
+
+import pytest
+
+from app.features.memoir.memoir_images.prompting import MemoirImagePromptService
+from app.features.memoir.memoir_images.settings import MemoirImageSettings
+
+
+def _svc(llm=None) -> MemoirImagePromptService:
+    return MemoirImagePromptService(llm=llm, settings=MemoirImageSettings())
+
+
+def test_story_primary_fallback_uses_placeholder_when_llm_disabled():
+    from app.features.memoir.memoir_images.image_placeholder_template import (
+        IMAGE_PLACEHOLDER_TEMPLATE,
+    )
+
+    out = _svc(llm=None).build_story_primary_prompt(
+        "My Story", "career", "rainy street", None
+    )
+    assert IMAGE_PLACEHOLDER_TEMPLATE in out["prompt"]
+    assert "rainy street" in out["prompt"]
+
+
+def test_story_primary_fallback_disabled_requires_brief(monkeypatch):
+    monkeypatch.setattr(
+        "app.features.memoir.memoir_images.prompting.settings.image_prompt_fallback_disabled",
+        True,
+    )
+    with pytest.raises(RuntimeError, match="prompt_brief"):
+        _svc(llm=None).build_story_primary_prompt("T", "career", "", None)
+
+
+def test_story_primary_style_from_chapter_category_when_no_intent_style():
+    """Story.stage 存章节 category 时，未指定 style_profile 应走 CATEGORY_STYLE_MAP。"""
+    out = _svc(llm=None).build_story_primary_prompt("T", "career_early", "brief", None)
+    assert out["style"] == "realistic"
+
+
+def test_story_primary_style_from_chat_stage_when_no_intent_style():
+    out = _svc(llm=None).build_story_primary_prompt("T", "career", "brief", None)
+    assert out["style"] == "realistic"
+
+
+def test_story_primary_fallback_disabled_requires_llm(monkeypatch):
+    monkeypatch.setattr(
+        "app.features.memoir.memoir_images.prompting.settings.image_prompt_fallback_disabled",
+        True,
+    )
+    with pytest.raises(RuntimeError, match="requires LLM"):
+        _svc(llm=None).build_story_primary_prompt("T", "career", "brief", None)
+
+
+def test_story_primary_llm_path(monkeypatch):
+    class _FakeLlm:
+        def invoke(self, *_a, **_kw):  # noqa: ANN001
+            raise AssertionError("invoke should not be used")
+
+    captured: dict[str, str] = {}
+
+    def fake_invoke_json(llm, prompt_text, **_kw):  # noqa: ANN001
+        captured["prompt_text"] = prompt_text
+        return json.dumps(
+            {
+                "prompt": "A hero scene at dawn",
+                "style": "watercolor",
+                "size": "1280x720",
+            }
+        )
+
+    monkeypatch.setattr(
+        "app.features.memoir.memoir_images.prompting.invoke_json_object",
+        fake_invoke_json,
+    )
+    out = _svc(llm=_FakeLlm()).build_story_primary_prompt(
+        "Title", "childhood", "memo", "realistic"
+    )
+    assert out["prompt"].lower().startswith("watercolor")
+    assert "dawn" in out["prompt"].lower()
+    assert out["style"] == "watercolor"
+    raw = captured.get("prompt_text", "")
+    assert "Title" in raw and "memo" in raw
+
+
+def test_cover_fallback_uses_service_template_when_llm_disabled():
+    out = _svc(llm=None).build_cover_prompt(
+        chapter_title="Ch1",
+        chapter_category="family",
+        context_excerpt="mountain lake",
+    )
+    assert "mountain lake" in out["prompt"].lower()
+    assert (
+        "chapter cover" in out["prompt"].lower()
+        or "illustration" in out["prompt"].lower()
+    )
+
+
+def test_cover_fallback_when_excerpt_empty(monkeypatch):
+    monkeypatch.setattr(
+        "app.features.memoir.memoir_images.prompting.settings.image_prompt_fallback_disabled",
+        False,
+    )
+    out = _svc(llm=None).build_cover_prompt(
+        chapter_title="",
+        chapter_category="family",
+        context_excerpt="",
+    )
+    assert len(out["prompt"]) > 20
+
+
+def test_cover_fallback_disabled_requires_excerpt(monkeypatch):
+    monkeypatch.setattr(
+        "app.features.memoir.memoir_images.prompting.settings.image_prompt_fallback_disabled",
+        True,
+    )
+    with pytest.raises(RuntimeError, match="context_excerpt"):
+        _svc(llm=None).build_cover_prompt(
+            chapter_title="T",
+            chapter_category="family",
+            context_excerpt="",
+        )