docs: add cascade delete note to db-schema-spec and improve delete_comment in postgres.py

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Co-authored-by: genichin <4282227+genichin@users.noreply.github.com>

app/repositories/postgres.py

@@ -1190,6 +1190,7 @@ class PostgresPortfolioRepository:
                 raise ValueError("포트폴리오를 찾을 수 없습니다")
             if clear_ledger:
                 cur.execute("DELETE FROM portfolio_ledger WHERE portfolio_id = %s", (portfolio_id,))
+            # Comments are always removed with the portfolio (no FK constraint, handled in application layer)
             cur.execute("DELETE FROM portfolio_comments WHERE portfolio_id = %s", (portfolio_id,))
             cur.execute("DELETE FROM portfolios WHERE portfolio_id = %s", (portfolio_id,))
 

docs/db-schema-spec.md

@@ -306,6 +306,8 @@ Current baseline intentionally has no DB-level FK constraints.
 
 - Logical relations are managed by application/repository logic.
 - When adding FK constraints in future migrations, update this document and include migration IDs.
+- Cascade behavior implemented in application layer:
+  - Deleting a portfolio also deletes all associated `portfolio_comments` rows (hard delete, no FK constraint).
 
 ## 8. Change Management Rule