feat(content): 发布体系 大/小图 + 公开 + 园区端发布

- 迁移0018：content_items 加 card_mode/is_public/source/tenant_id
- ContentRepository：_to_dict/create/update 含新字段；list 加 public_only/tenant_id 过滤
- /opc/content 仅 published+公开；/opc/content/{id} 非公开→404
- 园区端 POST /park/api/content：发布人强制=园区名、source=carrier、tenant=本园、is_public 默认false(仅大屏)、card_mode默认small
- 园区端 GET /park/api/content：本园区内容(含未公开)供大屏
- schemas 补 card_mode/is_public

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

alembic/versions/0018_content_publish.py

@@ -0,0 +1,29 @@
+"""资讯发布体系：card_mode/is_public/source/tenant_id
+
+Revision ID: 0018_content_publish
+Revises: 0017_content_detail
+Create Date: 2026-08-27
+"""
+from __future__ import annotations
+
+import sqlalchemy as sa
+from alembic import op
+
+revision = "0018_content_publish"
+down_revision = "0017_content_detail"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column("content_items", sa.Column("card_mode", sa.String(), nullable=False, server_default="big"))
+    op.add_column("content_items", sa.Column("is_public", sa.Boolean(), nullable=False, server_default="1"))
+    op.add_column("content_items", sa.Column("source", sa.String(), nullable=False, server_default="operator"))
+    op.add_column("content_items", sa.Column("tenant_id", sa.String(), nullable=False, server_default=""))
+
+
+def downgrade() -> None:
+    op.drop_column("content_items", "tenant_id")
+    op.drop_column("content_items", "source")
+    op.drop_column("content_items", "is_public")
+    op.drop_column("content_items", "card_mode")