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/alembic/versions/018_fission_notes_table.py

@@ -0,0 +1,57 @@
+"""create fission_notes table
+
+Revision ID: 018
+Revises: 017
+Create Date: 2026-06-18
+
+裂变重写（对齐上线版 split.js）：一次LLM出N套完整笔记包，每套落一行。
+每套含 标题/正文/标签/封面/imagePlan/裂变维度，带评分+生图结果。
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "018"
+down_revision = "017"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.create_table(
+        "fission_notes",
+        sa.Column("id", sa.BigInteger, primary_key=True, autoincrement=True),
+        sa.Column(
+            "fission_id", sa.BigInteger,
+            sa.ForeignKey("fission_tasks.id", ondelete="CASCADE"),
+            nullable=False,
+        ),
+        sa.Column(
+            "workspace_id", sa.BigInteger,
+            sa.ForeignKey("workspaces.id", ondelete="CASCADE"),
+            nullable=False, comment="多租户隔离",
+        ),
+        sa.Column("seq", sa.Integer, nullable=False, server_default="0",
+                  comment="第几套（0起）"),
+        sa.Column("note_json", sa.Text, nullable=True, comment="完整笔记包JSON"),
+        sa.Column("images_json", sa.Text, nullable=True, comment="生图结果JSON"),
+        sa.Column("score", sa.Integer, nullable=False, server_default="0"),
+        sa.Column("passed", sa.Integer, nullable=False, server_default="0",
+                  comment="是否过线(>=80)"),
+        sa.Column("needs_optimization", sa.Integer, nullable=False,
+                  server_default="0", comment="不达标降级草稿标记"),
+        sa.Column("dimension", sa.String(64), nullable=True,
+                  comment="裂变维度: 换角度/换痛点/换人群"),
+        sa.Column("status", sa.String(20), nullable=False,
+                  server_default="pending",
+                  comment="状态: pending/scored/image_done/failed"),
+        sa.Column("created_at", sa.DateTime, server_default=sa.func.now(),
+                  nullable=False),
+    )
+    op.create_index("idx_fission_notes_fission_id", "fission_notes", ["fission_id"])
+    op.create_index("idx_fission_notes_workspace_id", "fission_notes", ["workspace_id"])
+
+
+def downgrade():
+    op.drop_index("idx_fission_notes_workspace_id", table_name="fission_notes")
+    op.drop_index("idx_fission_notes_fission_id", table_name="fission_notes")
+    op.drop_table("fission_notes")