feat(models): add series column to Topic for content columns

Topic model: new series VARCHAR field (optional, indexed)
Schema: series in TopicBase/TopicCreate/TopicUpdate
API: GET /api/topics supports ?series= query param
DB migration: ALTER TABLE topics ADD COLUMN series

Ultraworked with Sisyphus

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

platform/backend/app/models.py

@@ -265,6 +265,7 @@ class Topic(Base):
     platform_urls = Column(JSON, default=dict)
     lock_by = Column(String, nullable=True)
     lock_at = Column(DateTime, nullable=True)
+    series = Column(String, nullable=True, index=True, comment="所属栏目，如：AI工具实测周报、科技人文思辨")
 
     field = relationship("TopicField", backref="topics")