refactor(api,expo): 多智能体与会话收敛、回忆录兼容层移除、后端测试集大幅删减

- 对齐「多智能体收敛」与「回忆录 stories-first / markdown-first」方向：收紧运行时契约、
  删除过渡兼容路径与双轨逻辑，并同步更新客户端与文档。

- Chat：以 ChatOrchestrator 为实时编排入口；删除独立 conversation_agent，精简 prompts。
- Memoir：删除 memory_agent；MemoirOrchestrator、classification / story_route 与 prompts 收敛到
  prepare_batches + run_story_pipeline_for_category_batch 主链路。
- 将 agents 侧 processor 迁入 feature 层为 background_runner，并移除 features 下重复/过时
  processor 封装。

- 新增 history_store，强化「conversation_messages 为 DB 真源、Redis 为缓存」模型。
- 调整 models、repo、service、session_history；精简 WS message_types，重构 pipeline 与 router。

- 移除章节占位、整章再生等旧路径；章节列表与封面逻辑要求 story 关联；收紧 cover 资格与
  enqueue。
- helpers、repo、service、router、reading_segment_materialize、story_pipeline_sync、pdf_service
  等按 canonical markdown / cover_asset_id 收缩；删除 memoir_images/provider 等冗余。
- tasks：memoir_tasks、chapter_cover_tasks 等大幅瘦身；story_image_tasks 等与当前图片任务对齐。

- core：config、logging、redis、task_tracker 小幅调整。
- auth / user / payment / quota：路由或服务侧删减过时接口或逻辑（如 payment router 行数减少）。

- pyproject.toml、development.sh、.env.example / .env.production、README 等同步说明或变量。

- Alembic 0001_initial_schema 微调（与当前 schema 叙事一致的小改动）。

- 回忆录：types / mappers / api、章节页与 memoir 页与后端契约对齐；markdown-renderer 调整。
- 语音：删除 voice/player，voice-segment-store 相应精简。

- api/tests：删除 conftest 及绝大部分既有测试文件（websocket_baseline、conversation、memoir
  图片、PDF、SMS 等），属有意收缩/待按 backend-test-system 重建的信号。
- docs：新增多智能体收敛与移除兼容层计划摘要；更新 story-first 设计、backend-test-system、
  multi-agent-refactor-plan、实施总结等。

BREAKING CHANGE: 后端对外契约、回忆录章节字段与若干路由/任务行为已变更；大量 API 测试被移除，
  CI 若依赖这些用例需按新策略补测或调整流水线。

api/app/features/conversation/service.py

@@ -16,7 +16,9 @@ from app.core.redis import redis_service
 from app.core.storage_purge import delete_object_storage_keys_best_effort
 from app.features.conversation import repo
 from app.features.conversation.models import Conversation
-from app.features.conversation.session_history import segments_to_redis_history
+from app.features.conversation.session_history import (
+    conversation_messages_to_redis_history,
+)
 from app.features.memory import repo as memory_repo
 from app.features.quota.service import QuotaService
 from app.ports.storage import ObjectStorage
@@ -104,32 +106,34 @@ class ConversationService:
         self._quota = quota_service
         self._object_storage = object_storage
 
-    async def _get_history(self, conversation_id: str) -> list[dict]:
-        return await redis_service.get_conversation_history(conversation_id)
-
     async def _clear_history(self, conversation_id: str) -> None:
         try:
             await redis_service.clear_conversation_history(conversation_id)
         except Exception:
             pass
 
-    async def ensure_redis_history_from_segments(
-        self, conversation_id: str
-    ) -> list[dict]:
+    async def ensure_redis_history_from_db(self, conversation_id: str) -> list[dict]:
         """
-        供 WS 与 get_messages 使用：优先 Redis；若为空则用 DB segments 重建并写回。
-        会话层编排，非 Agent 职责（ChatOrchestrator 只读写已存在的 Redis 流）。
+        供 WS 与 get_messages 使用：优先 Redis；若为空则用 DB conversation_messages 重建并写回。
         """
-        history = await redis_service.get_conversation_history(conversation_id)
+        try:
+            history = await redis_service.get_conversation_history(conversation_id)
+        except Exception as exc:
+            logger.warning("conversation history cache read skipped: %s", exc)
+            history = []
         if history:
             return history
-        segments = await repo.get_segments_for_conversation(conversation_id, self._db)
-        if not segments:
-            return []
-        rebuilt = segments_to_redis_history(segments)
-        if rebuilt:
-            await redis_service.set_conversation_history(conversation_id, rebuilt)
-        return rebuilt
+
+        rows = await repo.get_conversation_messages(conversation_id, self._db)
+        if rows:
+            rebuilt = conversation_messages_to_redis_history(rows)
+            try:
+                await redis_service.set_conversation_history(conversation_id, rebuilt)
+            except Exception as exc:
+                logger.warning("conversation history cache write skipped: %s", exc)
+            return rebuilt
+
+        return []
 
     async def list_for_user(self, user_id: str) -> list[dict]:
         conversations = await repo.get_user_conversations(user_id, self._db)
@@ -137,7 +141,7 @@ class ConversationService:
         for conv in conversations:
             history: list[dict] = []
             try:
-                history = await self._get_history(conv.id)
+                history = await self.ensure_redis_history_from_db(conv.id)
             except Exception:
                 pass
             latest_message = history[-1].get("content", "")[:50] if history else None
@@ -243,7 +247,7 @@ class ConversationService:
     async def get_messages(self, conversation_id: str, user_id: str) -> list[dict]:
         conv = await self.get_or_404(conversation_id, user_id)
         try:
-            history = await self.ensure_redis_history_from_segments(conversation_id)
+            history = await self.ensure_redis_history_from_db(conversation_id)
             return _build_messages_from_history(
                 conversation_id=conversation_id,
                 history=history,