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/reading_segment_materialize.py

@@ -1,7 +1,4 @@
-"""
-章节阅读片段物化：与 canonical 同一生成时机写入 reading_segments_json（无签名 URL）；
-API 读时 hydrate 或（dirty / 无快照）回退为运行时物化。
-"""
+"""章节阅读片段物化：与 canonical 同一生成时机写入 reading_segments_json（无签名 URL）。"""
 
 from __future__ import annotations
 
@@ -100,52 +97,12 @@ def build_reading_segments_snapshot(ch: Chapter) -> list[dict[str, Any]]:
             {
                 "story_id": st.id,
                 "body_markdown": body,
-                "cover_image": cover,
+                "cover_asset": cover,
             }
         )
     return out
 
 
-def materialize_chapter_reading_segments(
-    ch: Chapter,
-    asset_url_map: dict[str, str] | None = None,
-) -> list[dict[str, Any]]:
-    """运行时物化（解析签名 URL），与旧 build_reading_segments 行为一致。"""
-    from app.features.memoir import helpers as h
-
-    asset_url_map = asset_url_map or {}
-    resolve = lambda aid: asset_url_map.get(aid)  # noqa: E731
-    links = sorted(
-        list(getattr(ch, "story_links", None) or []),
-        key=lambda x: getattr(x, "order_index", 0),
-    )
-    segments: list[dict[str, Any]] = []
-    for link in links:
-        st = getattr(link, "story", None)
-        if st is None:
-            continue
-        title = (getattr(st, "title", None) or "").strip()
-        raw = (getattr(st, "canonical_markdown", None) or "").strip()
-        body = sanitize_story_for_chapter_compose(raw, title)
-        if not body:
-            continue
-        body_md = resolve_asset_refs_in_markdown(body, resolve)
-        img_raw = h.story_primary_cover_image_dict(st, asset_url_map=asset_url_map)
-        primary_aid = _primary_story_intent_asset_id(st)
-        inline_ids = set(collect_asset_ids_from_markdown(body))
-        if img_raw and primary_aid and primary_aid in inline_ids:
-            img_raw = None
-        img_norm = h.first_normalized_image_for_api(img_raw) if img_raw else None
-        segments.append(
-            {
-                "story_id": st.id,
-                "body_markdown": body_md,
-                "cover_image": img_norm,
-            }
-        )
-    return segments
-
-
 def hydrate_reading_segments_from_snapshot(
     ch: Chapter,
     asset_url_map: dict[str, str] | None = None,
@@ -159,7 +116,7 @@ def hydrate_reading_segments_from_snapshot(
     out: list[dict[str, Any]] = []
     for row in rows:
         body = resolve_asset_refs_in_markdown(row["body_markdown"], resolve)
-        ci = row.get("cover_image")
+        ci = row.get("cover_asset")
         if ci:
             img_raw = _cover_dict_from_snapshot_row(ci, asset_url_map)
             img_norm = h.first_normalized_image_for_api(img_raw)
@@ -169,7 +126,7 @@ def hydrate_reading_segments_from_snapshot(
             {
                 "story_id": row["story_id"],
                 "body_markdown": body,
-                "cover_image": img_norm,
+                "cover_asset": img_norm,
             }
         )
     return out
@@ -179,10 +136,8 @@ def resolve_reading_segments_for_chapter_detail(
     ch: Chapter,
     asset_url_map: dict[str, str] | None = None,
 ) -> list[dict[str, Any]]:
-    """章节详情：dirty 或无快照列时运行时物化；否则 hydrate。"""
+    """章节详情：仅读取已物化快照。"""
     asset_url_map = asset_url_map or {}
-    dirty = getattr(ch, "markdown_compose_dirty", True)
-    has_snapshot = getattr(ch, "reading_segments_json", None) is not None
-    if has_snapshot and not dirty:
-        return hydrate_reading_segments_from_snapshot(ch, asset_url_map=asset_url_map)
-    return materialize_chapter_reading_segments(ch, asset_url_map=asset_url_map)
+    if getattr(ch, "reading_segments_json", None) is None:
+        return []
+    return hydrate_reading_segments_from_snapshot(ch, asset_url_map=asset_url_map)