第11环裂变重写：对齐上线版 split.js 一次LLM出N套完整笔记包

架构从"扇出N个GenerationTask各跑完整管道"改为"一次LLM调用直接出N套
完整笔记包(N=1~5)"，落 FissionNote 表 + 独立展示页。

后端：
- 018迁移：fission_notes 表(文案JSON+score+passed+imagePlan+images+status)
- fission_prompt：FISSION_SYSTEM+三档参考度(low/mid/high)+normalize_tags+品类兜底
- fission_pipeline：一次LLM出N套→各评分(@80合格线)→排序→落库,不达标标
  needs_optimization 非丢弃；apiports 503 回落 codeproxy gpt-5.5 强档兜底
- fission_images：每套串行调现有生图接口(零改动image_gen/storyboard)
- tasks.py：run_fission_pipeline Celery task，删旧扇出注入
- api/v1/fission：进度聚合FissionNote + GET /fission/{id}/notes(剥内部字段)

前端：FissionProgress对齐状态机 + N套独立展示页 + FissionNoteCard

测试：test_fission_engine(19)+test_fission_pipeline(5) 全过；104 全量回归绿

实测task5(fanout=2,mid)端到端跑通：一次出2套→seq0=85过/seq1=79标优化→
生图codeproxy/edits→1024×1536去AI化→task completed→notes端点返完整数据

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/models/fission.py

@@ -51,3 +51,53 @@ class FissionTask(Base):
     __table_args__ = (
         Index("idx_fission_tasks_workspace_id", "workspace_id"),
     )
+
+
+class FissionNote(Base):
+    """裂变产出的单套完整笔记包（一次LLM出N套，每套落一行）。
+
+    对齐上线版 split.js handleContentSplit：每套含 标题/正文/标签/封面/imagePlan，
+    带评分+裂变维度+生图结果。北哥在独立展示页查看。
+    """
+    __tablename__ = "fission_notes"
+
+    id: Mapped[int] = mapped_column(BigInteger, primary_key=True, autoincrement=True)
+    fission_id: Mapped[int] = mapped_column(
+        BigInteger, ForeignKey("fission_tasks.id", ondelete="CASCADE"), nullable=False
+    )
+    workspace_id: Mapped[int] = mapped_column(
+        BigInteger, ForeignKey("workspaces.id", ondelete="CASCADE"), nullable=False
+    )
+    seq: Mapped[int] = mapped_column(
+        Integer, default=0, nullable=False, comment="第几套（0起）"
+    )
+    # 完整笔记包 JSON：title/content/tags/coverTitle/imagePlan/dimension/audience/scene/painPoint/keywords
+    note_json: Mapped[str | None] = mapped_column(
+        Text, comment="完整笔记包JSON"
+    )
+    # 生图结果 JSON：[{role,name,image_url,error}...]
+    images_json: Mapped[str | None] = mapped_column(
+        Text, comment="生图结果JSON"
+    )
+    score: Mapped[int] = mapped_column(Integer, default=0, nullable=False)
+    passed: Mapped[bool] = mapped_column(
+        Integer, default=0, nullable=False, comment="是否过线(>=80)"
+    )
+    needs_optimization: Mapped[bool] = mapped_column(
+        Integer, default=0, nullable=False, comment="不达标降级草稿标记"
+    )
+    dimension: Mapped[str | None] = mapped_column(
+        String(64), comment="裂变维度: 换角度/换痛点/换人群"
+    )
+    status: Mapped[str] = mapped_column(
+        String(20), default="pending", nullable=False,
+        comment="状态: pending/scored/image_done/failed"
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime, server_default=func.now(), nullable=False
+    )
+
+    __table_args__ = (
+        Index("idx_fission_notes_fission_id", "fission_id"),
+        Index("idx_fission_notes_workspace_id", "workspace_id"),
+    )