feat: 预设素材库 - 4 套主题 × 4 张图 = 16 张 PNG

主题：classical 古典宫廷（王子/皇后/国王/小丑）
      modern    现代人物（小孩/女青年/男青年/小丑鱼）
      astronomy 天文（星星/月亮/太阳/黑洞）
      minimal   简笔符号（圆点/♀/♂/叉）

改动：
- 新增 LibraryAsset 模型（全局素材库，theme_id/role/asset_id 索引）
- 新增 seed_library 管理命令，用 Pillow 画 16 张 PNG 素材
- 新增 /api/projects/library/ 列表 API
- 新增 /api/projects/{pid}/library/{id}/apply/ 应用 API
  把预设素材复制到 projects/<pid>/joker 或 face_card 下，作为该牌位的素材
- AssetPanel 加 tab 切换：「我的素材」+「预设主题」，主题可筛选、点击套用到当前牌
- 修复 generate_card_png 的 joker asset 匹配 bug：which 应该是 card_key（含前缀）才能匹配 asset_key

设计要点：
- 预设素材只画上半身（y=0~150），下半留空，让系统的'自动对称'流水线正确工作
- 预设素材 PNG 200×300，深色描边 + 半透明浅色填充，在任意牌面背景上叠加都清晰

backend/apps/projects/views_library.py

@@ -0,0 +1,114 @@
+from rest_framework.decorators import api_view
+from rest_framework.response import Response
+from rest_framework import status
+from django.conf import settings
+from .models import LibraryAsset, Project, Asset
+from .serializers import LibraryAssetSerializer
+import os
+
+
+@api_view(['GET'])
+def library_list(request):
+    """列出所有预设素材（按主题分组）"""
+    assets = LibraryAsset.objects.all()
+    serializer = LibraryAssetSerializer(assets, many=True, context={'request': request})
+
+    # 按 theme_id 分组
+    grouped = {}
+    for a in serializer.data:
+        a['file_url'] = f'{settings.MEDIA_URL}{a["file_path"]}'
+        grouped.setdefault(a['theme_id'], {
+            'theme_id': a['theme_id'],
+            'theme_name': a['theme_name'],
+            'description': a['description'],
+            'items': [],
+        })['items'].append(a)
+
+    return Response(list(grouped.values()))
+
+
+@api_view(['GET'])
+def library_themes(request):
+    """返回所有主题的元信息（不含具体素材项，用于渲染主题筛选器）"""
+    themes = LibraryAsset.objects.values('theme_id', 'theme_name', 'description').distinct()
+    return Response(list(themes))
+
+
+@api_view(['GET'])
+def library_detail(request, pk):
+    """单个预设素材的详情"""
+    try:
+        a = LibraryAsset.objects.get(pk=pk)
+    except LibraryAsset.DoesNotExist:
+        return Response({'error': 'Not found'}, status=status.HTTP_404_NOT_FOUND)
+    data = LibraryAssetSerializer(a, context={'request': request}).data
+    data['file_url'] = f'{settings.MEDIA_URL}{data["file_path"]}'
+    return Response(data)
+
+
+@api_view(['POST'])
+def library_apply(request, project_pk, pk):
+    """把预设素材应用到项目某张牌（默认：spade-J / joker-big 等）
+    请求体: { card_key?: 'spade-J' }  可选；不传则用预设素材的 role + 默认 spade
+    """
+    try:
+        project = Project.objects.get(pk=project_pk)
+        lib = LibraryAsset.objects.get(pk=pk)
+    except (Project.DoesNotExist, LibraryAsset.DoesNotExist):
+        return Response({'error': 'not found'}, status=status.HTTP_404_NOT_FOUND)
+
+    card_key = request.data.get('card_key')
+    if not card_key:
+        # 默认: spade-{role} 或 joker-{which}
+        if lib.role == 'joker':
+            card_key = 'joker-big'  # 默认应用到 big；用户可换
+        else:
+            card_key = f'spade-{lib.role}'
+
+    # 决定 asset_type: J/Q/K -> face_card；joker -> joker
+    asset_type = 'face_card' if lib.role in ('J', 'Q', 'K') else 'joker'
+    asset_key = card_key
+
+    # 删除该项目同 (asset_type, asset_key) 的旧记录（避免重复）
+    Asset.objects.filter(project=project, asset_type=asset_type, asset_key=asset_key).delete()
+
+    # 复制 library 文件到 projects/<pid>/ 下，避免污染原文件
+    import shutil
+    from time import time
+    project_media_dir = os.path.join('projects', str(project.id), asset_type)
+    full_dir = os.path.join(settings.MEDIA_ROOT, project_media_dir)
+    os.makedirs(full_dir, exist_ok=True)
+
+    src_path = os.path.join(settings.MEDIA_ROOT, lib.file_path)
+    ts = int(time() * 1000)
+    new_file_name = f'{asset_key}_{ts}_{lib.file_name}'
+    dst_rel = os.path.join(project_media_dir, new_file_name)
+    dst_abs = os.path.join(settings.MEDIA_ROOT, dst_rel)
+    shutil.copy2(src_path, dst_abs)
+
+    # 读 svg 尺寸
+    width = height = None
+    try:
+        from PIL import Image
+        with Image.open(dst_abs) as im:
+            width, height = im.size
+    except Exception:
+        pass
+
+    asset = Asset.objects.create(
+        project=project,
+        asset_type=asset_type,
+        asset_key=asset_key,
+        file_path=dst_rel,
+        file_name=new_file_name,
+        width=width,
+        height=height,
+    )
+
+    return Response({
+        'ok': True,
+        'asset_id': asset.id,
+        'card_key': card_key,
+        'asset_type': asset_type,
+        'file_url': f'{settings.MEDIA_URL}{dst_rel}',
+    }, status=status.HTTP_201_CREATED)