feat: REQ-005 assets table tag apply/remove SQL alignment (BAT-013)

- REQ-005 CHG-20260521-02 follow-up: clarify AC-1/AC-5 to target assets table
- Add append_asset_tag_id/remove_asset_tag_id on PostgresPortfolioRepository
- Replace internal calls in apply_asset_tag/remove_asset_tag routes (HTTP contract unchanged)
- 7 new regression tests: method existence, SQL target table, error branches, idempotency guard
- Update existing AC-1/AC-5 assertions to verify new methods + portfolios methods uncalled
- Documentation: runtime-flows.md flow body + change log; rel-001 includes BAT-013
- REQ-005 transition: Approved → Implemented (CHG-20260521-03) via BAT-013 verification
- Total tests: 362 → 369 passed

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/batches/index.md

@@ -20,3 +20,4 @@
 | BAT-010 | standard | release-candidate | REQ-030, REQ-031, REQ-032 | dcy-003 | [bat-010](./bat-010_20260521_v1-portfolios-sub-resources/) |
 | BAT-011 | standard | release-candidate | REQ-029 | dcy-003 | [bat-011](./bat-011_20260521_v1-portfolios-ledger-path-migration/) |
 | BAT-012 | standard | release-candidate | REQ-002, REQ-010 | dcy-001, dcy-002, dcy-003 | [bat-012](./bat-012_20260521_assets-tagging-and-admin-get-redefinition/) |
+| BAT-013 | batch-lite | release-candidate | REQ-005 | dcy-002 | [bat-013](./bat-013_20260521_asset-tag-apply-remove-fix/) |