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/0009_story_image_intent_constraints.py

@@ -0,0 +1,72 @@
+"""story_image_intents: one primary per story + optional FK to assets
+
+Revision ID: 0009_si_constraints
+Revises: 0008_legacy_assets
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0009_si_constraints"
+down_revision: Union[str, Sequence[str], None] = "0008_legacy_assets"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    bind = op.get_bind()
+    bind.execute(
+        sa.text(
+            """
+            UPDATE story_image_intents SET asset_id = NULL
+            WHERE asset_id IS NOT NULL
+              AND NOT EXISTS (SELECT 1 FROM assets a WHERE a.id = story_image_intents.asset_id)
+            """
+        )
+    )
+    # 去重：同一 story 多条 primary 时保留最新一条
+    bind.execute(
+        sa.text(
+            """
+            DELETE FROM story_image_intents
+            WHERE id IN (
+              SELECT id FROM (
+                SELECT id,
+                  ROW_NUMBER() OVER (
+                    PARTITION BY story_id
+                    ORDER BY created_at DESC NULLS LAST, id DESC
+                  ) AS rn
+                FROM story_image_intents
+                WHERE intent_role = 'primary'
+              ) t
+              WHERE rn > 1
+            )
+            """
+        )
+    )
+    op.create_foreign_key(
+        "fk_story_image_intents_asset_id_assets",
+        "story_image_intents",
+        "assets",
+        ["asset_id"],
+        ["id"],
+        ondelete="SET NULL",
+    )
+    op.execute(
+        """
+        CREATE UNIQUE INDEX IF NOT EXISTS uq_story_primary_image_intent
+        ON story_image_intents (story_id)
+        WHERE intent_role = 'primary'
+        """
+    )
+
+
+def downgrade() -> None:
+    op.execute("DROP INDEX IF EXISTS uq_story_primary_image_intent")
+    op.drop_constraint(
+        "fk_story_image_intents_asset_id_assets",
+        "story_image_intents",
+        type_="foreignkey",
+    )