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>

alembic/versions/0016_content_media.py

@@ -0,0 +1,35 @@
+"""资讯富字段：封面/图集/视频/外链/发布人
+
+Revision ID: 0016_content_media
+Revises: 0015_company_compute
+Create Date: 2026-08-27
+"""
+from __future__ import annotations
+
+import sqlalchemy as sa
+from alembic import op
+
+revision = "0016_content_media"
+down_revision = "0015_company_compute"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column("content_items", sa.Column("cover", sa.String(), nullable=False, server_default=""))
+    op.add_column("content_items", sa.Column("images_json", sa.Text(), nullable=False, server_default="[]"))
+    op.add_column("content_items", sa.Column("video", sa.String(), nullable=False, server_default=""))
+    op.add_column("content_items", sa.Column("video_cover", sa.String(), nullable=False, server_default=""))
+    op.add_column("content_items", sa.Column("link", sa.String(), nullable=False, server_default=""))
+    op.add_column("content_items", sa.Column("publisher_name", sa.String(), nullable=False, server_default=""))
+    op.add_column("content_items", sa.Column("publisher_avatar", sa.String(), nullable=False, server_default=""))
+
+
+def downgrade() -> None:
+    op.drop_column("content_items", "publisher_avatar")
+    op.drop_column("content_items", "publisher_name")
+    op.drop_column("content_items", "link")
+    op.drop_column("content_items", "video_cover")
+    op.drop_column("content_items", "video")
+    op.drop_column("content_items", "images_json")
+    op.drop_column("content_items", "cover")