feat(M7): 第11环裂变引擎端到端打通

- fission_service: 1源任务扇出N套子任务(回填source_fission_id,复用主管道),MAX_FANOUT=5红线
- fission_prompt: 三档参考度(high/mid/low)占位规则,真实措辞待北哥定义
- tasks.py: worker识别裂变子任务→注入对应档位规则进文案上下文(关键缺口修复)
- api/v1/fission: POST触发 + GET进度聚合, main.py注册路由
- 实测: fission_id=2扇出task70/71,worker注入日志level=high,进度端点聚合正确

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

backend/app/services/fission_service.py

@@ -0,0 +1,84 @@
+"""
+app/services/fission_service.py — 第11环裂变 扇出 service
+
+1爆款 → N套完整笔记包。每套=一个独立 GenerationTask(回填 source_fission_id),
+复用主管道 run_generation_pipeline 生成文案+配图。
+🔴 占位档位规则见 fission_prompt.py,真实措辞待北哥定义(倩倩姐2026-06-16先打通)。
+"""
+import json
+import logging
+
+from sqlalchemy.orm import Session
+
+from app.core.response import raise_business
+from app.middleware.workspace_guard import CurrentUser
+from app.models.fission import FissionTask
+from app.models.task import GenerationTask
+from app.services.ai_engine.fission_prompt import valid_level
+from app.services.task_service import _check_user_has_key, enqueue_generation
+
+logger = logging.getLogger(__name__)
+
+MAX_FANOUT = 5  # 每用户并发上限5(红线),裂变扇出数受同等约束
+
+
+def create_fission(
+    db: Session, current_user: CurrentUser,
+    source_task_id: int, reference_level: str, fanout_count: int,
+) -> tuple[int, list[int]]:
+    """
+    从一个源任务裂变出 N 套子任务。
+    返回 (fission_id, [子task_id,...])。
+    """
+    _check_user_has_key(db, current_user.user_id, current_user.workspace_id)
+
+    level = valid_level(reference_level)
+    n = max(1, min(fanout_count or 3, MAX_FANOUT))
+
+    # 读源任务(取产品+主题+已选文案作为爆款源)
+    src = db.query(GenerationTask).filter(
+        GenerationTask.id == source_task_id,
+        GenerationTask.workspace_id == current_user.workspace_id,
+    ).first()
+    if not src:
+        raise_business("裂变源任务不存在")
+
+    source_note = {"title": src.theme or "", "content": _extract_source_copy(db, src)}
+
+    # 建 fission_task 记录
+    ft = FissionTask(
+        workspace_id=current_user.workspace_id,
+        source_note=json.dumps(source_note, ensure_ascii=False),
+        reference_level=level, fanout_count=n, status="generating",
+    )
+    db.add(ft); db.commit(); db.refresh(ft)
+
+    # 扇出 N 个子任务(回填 source_fission_id),复用主管道
+    task_ids: list[int] = []
+    for _ in range(n):
+        sub = GenerationTask(
+            workspace_id=current_user.workspace_id,
+            product_id=src.product_id, operator_id=current_user.user_id,
+            theme=src.theme, text_count=src.text_count, image_count=src.image_count,
+            track="ai", need_product_image=src.need_product_image,
+            status="pending", source_fission_id=ft.id,
+        )
+        db.add(sub); db.commit(); db.refresh(sub)
+        enqueue_generation(sub.id)
+        task_ids.append(sub.id)
+
+    logger.info("fission created id=%s level=%s fanout=%s tasks=%s",
+                ft.id, level, n, task_ids)
+    return ft.id, task_ids
+
+
+def _extract_source_copy(db: Session, src: GenerationTask) -> str:
+    """取源任务已选/最高分文案当爆款正文;无则用主题兜底。"""
+    from app.models.task import TextCandidate
+    c = (db.query(TextCandidate)
+         .filter(TextCandidate.task_id == src.id)
+         .order_by(TextCandidate.is_selected.desc(), TextCandidate.id.asc())
+         .first())
+    if c and c.content:
+        return c.content
+    return src.theme or ""