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/memoir/repo.py

@@ -12,9 +12,6 @@ from app.features.memoir.asset_resolver import collect_asset_ids_for_chapter
 from app.features.memoir.chapter_markdown_compose import (
     materialize_chapter_markdown_from_loaded_chapter,
 )
-from app.features.memoir.reading_segment_materialize import (
-    build_reading_segments_snapshot,
-)
 from app.features.memoir.models import (
     Book,
     Chapter,
@@ -23,6 +20,9 @@ from app.features.memoir.models import (
     ChapterVersion,
     MemoirState,
 )
+from app.features.memoir.reading_segment_materialize import (
+    build_reading_segments_snapshot,
+)
 from app.features.story.models import Story
 
 
@@ -64,19 +64,6 @@ async def get_chapters_for_memoir_list(
     return list(result.unique().scalars().all())
 
 
-async def get_chapters_with_sections(
-    user_id: str,
-    db: AsyncSession,
-    *,
-    active_only: bool = True,
-    is_new_only: bool | None = None,
-) -> list[Chapter]:
-    """兼容旧名：与 get_chapters_for_memoir_list 相同。"""
-    return await get_chapters_for_memoir_list(
-        user_id, db, active_only=active_only, is_new_only=is_new_only
-    )
-
-
 async def get_chapter_by_id(chapter_id: str, db: AsyncSession) -> Chapter | None:
     stmt = (
         select(Chapter)
@@ -98,138 +85,6 @@ async def get_memoir_state(user_id: str, db: AsyncSession) -> MemoirState | None
     return result.scalar_one_or_none()
 
 
-def get_archived_chapter_summaries_sync(
-    session: Session, user_id: str, category: str
-) -> list[tuple[str, str]]:
-    """获取已删除（is_active=False）的同类别章节的标题与内容摘要，供 AI 参考。"""
-    stmt = (
-        select(Chapter)
-        .where(
-            Chapter.user_id == user_id,
-            Chapter.category == category,
-            Chapter.is_active == False,  # noqa: E712
-        )
-        .order_by(Chapter.updated_at.desc())
-    )
-    result = session.execute(stmt)
-    chapters = list(result.unique().scalars().all())
-    summaries: list[tuple[str, str]] = []
-    for ch in chapters:
-        combined = (getattr(ch, "canonical_markdown", None) or "").strip()
-        preview = (combined[:200] + "...") if len(combined) > 200 else combined
-        if preview.strip():
-            summaries.append((ch.title or "", preview))
-    return summaries
-
-
-def ensure_chapter_markdown_and_version_sync(
-    session: Session,
-    chapter: Chapter,
-    markdown: str,
-) -> None:
-    """
-    为已有 chapter 设置 canonical_markdown 并创建 chapter_version。
-    供非 story 物化路径（如 save_chapter_markdown_sync）使用。
-    """
-    from sqlalchemy import func
-
-    count_stmt = select(func.count(ChapterVersion.id)).where(
-        ChapterVersion.chapter_id == chapter.id
-    )
-    version_no = (session.execute(count_stmt).scalar() or 0) + 1
-
-    version = ChapterVersion(
-        id=str(uuid.uuid4()),
-        chapter_id=chapter.id,
-        version_no=version_no,
-        markdown_snapshot=markdown,
-        actor_type="ai",
-        source_type="generate",
-    )
-    session.add(version)
-    session.flush()
-    chapter.canonical_markdown = markdown
-    chapter.current_version_id = version.id
-
-
-def save_chapter_markdown_sync(
-    session: Session,
-    *,
-    user_id: str,
-    chapter_id: str | None,
-    title: str,
-    category: str,
-    order_index: int,
-    markdown: str,
-    source_segments: list[str] | None = None,
-) -> Chapter:
-    """
-    将 markdown 写入 chapter.canonical_markdown 和 chapter_versions。
-    Agent 不直接调用，由 service/task 调用。
-    若 chapter_id 为 None 则新建章节。
-    """
-    if chapter_id:
-        chapter = session.get(Chapter, chapter_id)
-        if not chapter or chapter.user_id != user_id:
-            raise ValueError(f"Chapter {chapter_id} not found or access denied")
-    else:
-        chapter = Chapter(
-            id=str(uuid.uuid4()),
-            user_id=user_id,
-            title=title,
-            category=category,
-            order_index=order_index,
-            status="completed",
-            is_new=True,
-            is_active=True,
-            source_segments=source_segments or [],
-        )
-        session.add(chapter)
-        session.flush()
-
-    # 创建 chapter_version
-    from sqlalchemy import func
-
-    count_stmt = select(func.count(ChapterVersion.id)).where(
-        ChapterVersion.chapter_id == chapter.id
-    )
-    version_no = (session.execute(count_stmt).scalar() or 0) + 1
-
-    version = ChapterVersion(
-        id=str(uuid.uuid4()),
-        chapter_id=chapter.id,
-        version_no=version_no,
-        markdown_snapshot=markdown,
-        actor_type="ai",
-        source_type="generate",
-    )
-    session.add(version)
-    session.flush()
-
-    chapter.canonical_markdown = markdown
-    chapter.current_version_id = version.id
-    chapter.title = title
-    chapter.is_new = True
-    if source_segments:
-        chapter.source_segments = list(
-            set((chapter.source_segments or []) + source_segments)
-        )
-
-    session.flush()
-    session.refresh(chapter)
-    return chapter
-
-
-async def count_chapter_story_links(db: AsyncSession, chapter_id: str) -> int:
-    stmt = (
-        select(func.count())
-        .select_from(ChapterStoryLink)
-        .where(ChapterStoryLink.chapter_id == chapter_id)
-    )
-    n = await db.scalar(stmt)
-    return int(n or 0)
-
-
 async def get_chapter_ids_linked_to_story(db: AsyncSession, story_id: str) -> list[str]:
     stmt = select(ChapterStoryLink.chapter_id).where(
         ChapterStoryLink.story_id == story_id