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/database.py

@@ -131,6 +131,7 @@ def init_db():
                 ("search_rankings", "org_id", "VARCHAR DEFAULT 'default'"),
                 ("geo_readiness_scores", "org_id", "VARCHAR DEFAULT 'default'"),
                 ("articles", "org_id", "VARCHAR DEFAULT 'default'"),
+                ("topics", "series", "VARCHAR"),
             ]:
                 try:
                     conn.execute(text(f"ALTER TABLE {table} ADD COLUMN IF NOT EXISTS {col} {typ}"))