feat: 强化多视角图片一致性 + 修复下载逻辑 + 技术文档

- 新增品类专属背面/侧面描述(BACK_VIEW_HINTS/SIDE_VIEW_HINTS)
- 强化一致性前缀策略，按视角定制相机位置描述
- 更新视角映射提示词为纯摄影术语
- 修复前端下载逻辑：改用fetch直接下载当前视角图片
- HTTPS改HTTP修复外网URL访问
- 新增多视角一致性与3D视频生成技术文档

backend/app/services/design_service.py

@@ -3,8 +3,11 @@
 处理设计相关的业务逻辑，支持 AI 多视角生图 + mock 降级
 """
 import os
+import uuid
 import logging
 from typing import List, Optional, Tuple
+
+import httpx
 from sqlalchemy.orm import Session
 from sqlalchemy import desc
 
@@ -150,8 +153,26 @@ async def _generate_ai_images(
         # 调用 AI 生图
         # 后续视角传入 seed（Kolors）或参考图 URL（Seedream）保持一致性
         ref_url = first_remote_url if idx > 0 else None
+
+        # 后续视角在提示词前加一致性约束前缀（根据视角名称定制）
+        final_prompt = prompt_text
+        if idx > 0:
+            # 根据视角名称生成更具体的一致性前缀
+            view_angle_map = {
+                "正面图": "moving the camera to face the object directly from the front",
+                "侧面图": "moving the camera 90 degrees to the left side of the object",
+                "背面图": "moving the camera 180 degrees to see the reverse/back side of the object",
+            }
+            angle_desc = view_angle_map.get(view_name, "changing the camera angle")
+            consistency_prefix = (
+                f"Photograph the EXACT SAME jade object from the reference image, {angle_desc}. "
+                "The object does NOT move or change - only the camera position changes. "
+                "The shape, size, color, material texture, and all physical features must remain IDENTICAL. "
+            )
+            final_prompt = consistency_prefix + prompt_text
+
         remote_url, returned_seed = await ai_generator.generate_image(
-            prompt_text, model, seed=shared_seed, ref_image_url=ref_url
+            final_prompt, model, seed=shared_seed, ref_image_url=ref_url
         )
 
         # 第一张图保存信息供后续视角复用
@@ -161,8 +182,9 @@ async def _generate_ai_images(
                 shared_seed = returned_seed
             logger.info(f"多视角生图: seed={shared_seed}, ref_url={remote_url[:60]}...")
 
-        # 直接使用远程 URL，不下载到本地（节省服务器存储空间）
-        image_url = remote_url
+        # 下载到本地持久化存储（远程URL是临时链接，会过期失效）
+        image_url = await _download_image_to_local(remote_url, design.id, idx)
+        logger.info(f"视角[{view_name}] 已下载到本地: {image_url}")
 
         # 创建 DesignImage 记录
         design_image = DesignImage(
@@ -182,6 +204,34 @@ async def _generate_ai_images(
     design.status = "completed"
 
 
+async def _download_image_to_local(remote_url: str, design_id: int, idx: int) -> str:
+    """
+    下载远程 AI 生成的图片到本地 uploads/designs/ 目录
+    第三方AI服务生成的图片URL是临时链接，会过期失效，必须下载到本地持久化
+
+    Returns:
+        本地图片 URL 路径，如 /uploads/designs/123_0_xxxx.png
+    """
+    designs_dir = os.path.join(settings.UPLOAD_DIR, "designs")
+    os.makedirs(designs_dir, exist_ok=True)
+
+    filename = f"{design_id}_{idx}_{uuid.uuid4().hex[:8]}.png"
+    local_path = os.path.join(designs_dir, filename)
+
+    try:
+        async with httpx.AsyncClient(timeout=60, follow_redirects=True) as client:
+            resp = await client.get(remote_url)
+            resp.raise_for_status()
+            with open(local_path, "wb") as f:
+                f.write(resp.content)
+            logger.info(f"图片下载完成: {len(resp.content)} 字节 -> {local_path}")
+    except Exception as e:
+        logger.error(f"图片下载失败，回退使用远程URL: {e}")
+        return remote_url  # 下载失败时回退使用远程URL
+
+    return f"/uploads/designs/{filename}"
+
+
 def _generate_mock_fallback(
     db: Session,
     design: Design,