重构回忆录为 story-first / markdown-first 架构并整合图片意图与前端 UI 修复

本次 squash merge 将 codex-story-first-image-intent 的整体改动合入 development，核心内容包括：

1. 后端数据与迁移：新增 stories、story_versions、story_image_intents、chapter_cover_intents、assets 等模型与 Alembic 迁移，建立 story-first、markdown-first、asset-first 的主数据链路。

2. 生成与任务链：引入 StoryBuilderOrchestrator、ChapterComposerOrchestrator、story_image_tasks、chapter_cover_tasks，图片生成从正文占位符改为结构化 intent -> asset -> markdown 回填。

3. 并发与一致性：为 story/chapter intent 增加 claim_token、claimed_at、attempt_count，采用数据库原子 claim 为主、Redis 锁为辅，避免重复生成、锁误删和 processing 卡死。

4. Memoir 读写路径：章节 canonical_markdown 成为正文真源，列表/详情接口补齐 markdown、cover_asset、word_count 等字段，PDF 与 asset 解析链路同步升级。

5. Memory / Retrieval：扩展 transcript ingest、chunking、evidence 检索与 story 聚合基础设施，为后续 story-first RAG 与多 agent 编排提供底座。

6. App 端体验：章节页继续走 MarkdownRenderer 阅读链，同时吸收 fix3-19 的跨平台 UI glitch 修复；更新对话页、首页、文案资源与章节列表映射逻辑。

7. 测试与文档：补充 asset resolver、story image task、章节封面派发、markdown 映射等回归测试，并加入图片占位符退役设计文档。

api/alembic/versions/0008_migrate_legacy_images_to_assets.py

@@ -0,0 +1,110 @@
+"""migrate legacy placeholders and memoir_images to assets
+
+1. 从 chapters.canonical_markdown 移除 {{IMAGE:...}} / {{{{IMAGE:...}}}} 占位符
+2. 将已完成 memoir_images（含 storage_key）写入 assets；章节封面绑定 cover_asset_id
+
+Revision ID: 0008_legacy_assets
+Revises: 0007_assets
+Create Date: 2026-03-19
+
+"""
+
+import uuid
+from datetime import datetime, timezone
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "0008_legacy_assets"
+down_revision: Union[str, Sequence[str], None] = "0007_assets"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    from app.features.memoir.asset_resolver import strip_legacy_image_placeholders
+
+    conn = op.get_bind()
+
+    rows_ch = conn.execute(
+        sa.text(
+            "SELECT id, canonical_markdown FROM chapters WHERE canonical_markdown IS NOT NULL"
+        )
+    ).fetchall()
+    for cid, md in rows_ch:
+        if not md or not str(md).strip():
+            continue
+        cleaned = strip_legacy_image_placeholders(str(md))
+        if cleaned != str(md).strip():
+            conn.execute(
+                sa.text("UPDATE chapters SET canonical_markdown = :md WHERE id = :id"),
+                {"md": cleaned, "id": cid},
+            )
+
+    rows_mi = conn.execute(
+        sa.text(
+            """
+            SELECT id, chapter_id, section_id, storage_key, url, provider, style, status
+            FROM memoir_images
+            WHERE status = 'completed'
+              AND storage_key IS NOT NULL
+              AND TRIM(storage_key) <> ''
+            """
+        )
+    ).fetchall()
+
+    existing = {
+        r[0]
+        for r in conn.execute(sa.text("SELECT storage_key FROM assets")).fetchall()
+        if r[0]
+    }
+
+    for row in rows_mi:
+        _mid, chapter_id, section_id, storage_key, url, provider, style, _status = row
+        sk = (storage_key or "").strip()
+        if not sk or sk in existing:
+            continue
+        aid = str(uuid.uuid4())
+        asset_type = "chapter_cover" if section_id is None else "story_image"
+        now = datetime.now(timezone.utc)
+        conn.execute(
+            sa.text(
+                """
+                INSERT INTO assets (
+                    id, asset_type, storage_key, url, provider, style_profile,
+                    prompt_final, status, width, height, created_at
+                ) VALUES (
+                    :id, :atype, :sk, :url, :prov, :style,
+                    :prompt, 'completed', NULL, NULL, :created
+                )
+                """
+            ),
+            {
+                "id": aid,
+                "atype": asset_type,
+                "sk": sk,
+                "url": url,
+                "prov": provider,
+                "style": style,
+                "prompt": None,
+                "created": now,
+            },
+        )
+        existing.add(sk)
+        if section_id is None and chapter_id:
+            conn.execute(
+                sa.text(
+                    """
+                    UPDATE chapters
+                    SET cover_asset_id = :aid
+                    WHERE id = :cid
+                      AND (cover_asset_id IS NULL OR cover_asset_id = '')
+                    """
+                ),
+                {"aid": aid, "cid": chapter_id},
+            )
+
+
+def downgrade() -> None:
+    pass