feat(content): 资讯富编辑(封面/图集/视频/外链/发布人) + 媒体上传

- 迁移0016：content_items 加 cover/images_json/video/video_cover/link/publisher_name/publisher_avatar
- ContentRepository：_to_dict/create/update 支持富字段；images(list)↔images_json；update 白名单含 publisher_id
- schemas：ContentCreate/UpdateRequest 加富字段
- rbac_operator：POST /content 注入发布人(actor 昵称/头像快照)；新增 POST /admin/upload(媒体上传)
- app/services/media_upload.py：统一上传(serverdata/uploads + abs_url)，与 /api/upload 同契约

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

app/infrastructure/models.py

@@ -298,6 +298,14 @@ class ContentItem(Base):
     summary: Mapped[str] = mapped_column(String, default="")
     body: Mapped[str] = mapped_column(Text, default="")
     publisher_id: Mapped[str | None] = mapped_column(ForeignKey("users.id"), nullable=True)
+    # 资讯富字段：封面/图集/视频/外链/发布人
+    cover: Mapped[str] = mapped_column(String, default="")            # 封面图 URL
+    images_json: Mapped[str] = mapped_column(Text, default="[]")      # 图集（JSON url 数组）
+    video: Mapped[str] = mapped_column(String, default="")            # 视频 URL
+    video_cover: Mapped[str] = mapped_column(String, default="")      # 视频封面
+    link: Mapped[str] = mapped_column(String, default="")             # 外链
+    publisher_name: Mapped[str] = mapped_column(String, default="")   # 发布人昵称快照
+    publisher_avatar: Mapped[str] = mapped_column(String, default="") # 发布人头像快照
     status: Mapped[str] = mapped_column(String, default="draft")  # draft/published/offline
     created_at: Mapped[str] = mapped_column(String, default="")
     updated_at: Mapped[str] = mapped_column(String, default="")