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/prompt_builder.py

@@ -29,6 +29,41 @@ CATEGORY_VIEWS: Dict[str, List[str]] = {
     "随形": ["效果图", "正面图", "侧面图", "背面图"],
 }
 
+# ============================================================
+# 品类专属背面描述（不同品类的背面特征差异很大）
+# ============================================================
+BACK_VIEW_HINTS: Dict[str, str] = {
+    "牌子": (
+        "IMPORTANT: The reverse/back side of a jade pendant plaque is traditionally a smooth, flat, polished surface. "
+        "It may have a brief inscription or seal mark, but it must NOT have any carved figure, face, or decorative relief pattern. "
+        "The back is plain and minimalist. Do NOT mirror or duplicate the front carving on the back."
+    ),
+    "手把件": (
+        "The back of this hand-held jade piece continues the same sculptural form as the front. "
+        "It is part of the same three-dimensional object, showing the natural continuation of the carving from the rear angle."
+    ),
+    "雕刻件": (
+        "The back of this carved jade piece shows the rear of the same three-dimensional sculpture. "
+        "The carving continues around the object naturally, not a separate or different design."
+    ),
+    "摆件": (
+        "The back of this jade display piece shows the rear of the same three-dimensional artwork. "
+        "The form and carving continue naturally around the object."
+    ),
+    "随形": (
+        "The back of this free-form jade piece shows the natural stone surface from the rear. "
+        "The organic shape continues naturally, the back may show more of the raw jade texture."
+    ),
+}
+
+# 品类专属侧面描述
+SIDE_VIEW_HINTS: Dict[str, str] = {
+    "牌子": (
+        "The side/edge view of a jade pendant plaque shows its thin, flat profile. "
+        "The plaque is typically 5-10mm thick, showing the edge thickness and any subtle edge carving."
+    ),
+}
+
 
 def _load_mappings(mapping_type: str) -> Dict[str, str]:
     """从数据库加载指定类型的映射字典"""
@@ -159,7 +194,13 @@ def build_prompt(
     view_desc = view_map.get(view_name, "three-quarter view")
     parts.append(view_desc)
 
-    # 12. 质量后缀
+    # 12. 品类专属视角描述（背面/侧面特征）
+    if view_name == "背面图" and category_name in BACK_VIEW_HINTS:
+        parts.append(BACK_VIEW_HINTS[category_name])
+    elif view_name == "侧面图" and category_name in SIDE_VIEW_HINTS:
+        parts.append(SIDE_VIEW_HINTS[category_name])
+
+    # 13. 质量后缀
     parts.append(quality_suffix)
 
     return ", ".join(parts)