feat(BAT-005): add broker credential CLI command with persistence and tests

- Add CredentialService for managing broker credentials
- Implement credential CLI commands (create, list, get, update, delete)
- Add postgres repository support for credential operations
- Add comprehensive test coverage for credential CLI
- Update docs/batches/index.md and docs/srs/index.md with BAT-005
- Update docs/project-structure.md to reflect credential service

app/repositories/postgres.py

@@ -1398,6 +1398,17 @@ class PostgresPortfolioRepository:
             row = cur.fetchone()
         return self._row_to_broker_credential(row)
 
+    def delete_broker_credential(self, user_id: str, broker: str) -> bool:
+        with self._connect().cursor() as cur:
+            cur.execute(
+                """
+                DELETE FROM broker_credentials
+                WHERE user_id = %s AND broker = %s
+                """,
+                (user_id, broker),
+            )
+            return cur.rowcount > 0
+
     # ------------------------------------------------------------------
     # Ledger trades
     # ------------------------------------------------------------------