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/api/topics.py

@@ -31,6 +31,7 @@ def list_topics(
     status: Optional[str] = None,
     today: Optional[bool] = None,
     tag: Optional[str] = None,
+    series: Optional[str] = None,
     search: Optional[str] = None,
     sort_by: str = Query("priority_score", enum=["priority_score", "created_at", "title", "updated_at"]),
     order: str = Query("desc", enum=["asc", "desc"]),
@@ -54,6 +55,8 @@ def list_topics(
         query = query.filter(func.date(Topic.created_at) == date.today())
     if tag:
         query = query.filter(Topic.tags.contains([tag]))
+    if series:
+        query = query.filter(Topic.series == series)
     if search:
         query = query.filter(Topic.title.contains(search))