裂变补图：中转站波动导致单张失败时只重生失败分镜

实测发现 codeproxy 偶发不稳，单套中某张图重试3次仍失败会记 error。
本次加按需补图：只重生标 error 的分镜，成功的图与 token 不浪费。

- 019迁移+模型：fission_tasks 加 source_task_id(新架构不扇出GenerationTask,
  补图需复刻首轮上下文:产品/operator key/张数,故落库回查)
- fission_image_retry：只补失败 role(引擎 target_role 单张重生,R2零改动),
  merge 回 images_json,全恢复则 status=image_done
- tasks.py：retry_fission_images Celery task,key归属源任务operator(基石B),
  幂等锁防重复补图
- api：POST /fission/{id}/notes/{seq}/retry-images,无失败分镜守卫 queued=false
- 前端：FissionNoteCard 失败提示改"重试补图"按钮+loading,父页轮询刷新

实测task5 seq1(hook图首轮codeproxy重试3次失败)补图跑通：只补hook(1024×1536
去AI化)、applied_proof原图不动、recovered=1 still_failed=0、status=image_done;
apiports503回落codeproxy gpt-5.5强档兜底;seq0无失败图守卫queued=false

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

backend/app/api/v1/fission.py

@@ -115,3 +115,44 @@ def get_fission_notes(
 
     return ok({"fission_id": ft.id, "status": ft.status,
                "fanout_count": ft.fanout_count, "notes": notes})
+
+
+@router.post("/fission/{fission_id}/notes/{seq}/retry-images")
+def retry_fission_note_images_endpoint(
+    fission_id: int,
+    seq: int,
+    current_user: Annotated[CurrentUser, Depends(require_write_permission)] = None,
+    db: Session = Depends(get_db),
+):
+    """补图：对某一套只重生标了 error 的失败分镜（中转站波动兜底）。
+
+    异步丢 Celery；前端轮询 GET /fission/{id}/notes 看 images 更新。
+    """
+    ft = db.query(FissionTask).filter(
+        FissionTask.id == fission_id,
+        FissionTask.workspace_id == current_user.workspace_id,
+    ).first()
+    if not ft:
+        raise_not_found("裂变任务不存在")
+
+    fn = db.query(FissionNote).filter(
+        FissionNote.fission_id == fission_id,
+        FissionNote.workspace_id == current_user.workspace_id,
+        FissionNote.seq == seq,
+    ).first()
+    if not fn:
+        raise_not_found("该套笔记不存在")
+
+    try:
+        images = json.loads(fn.images_json) if fn.images_json else []
+    except (ValueError, TypeError):
+        images = []
+    failed = [img.get("role") for img in images if img.get("error")]
+    if not failed:
+        return ok({"fission_id": fission_id, "seq": seq,
+                   "queued": False, "message": "该套无失败分镜，无需补图"})
+
+    from app.workers.tasks import retry_fission_images
+    retry_fission_images.delay(fission_id, fn.id)
+    return ok({"fission_id": fission_id, "seq": seq, "note_id": fn.id,
+               "queued": True, "failed_roles": failed})