docs: add SQL safety rules to post-implementation checklist

CLAUDE.md

@@ -91,6 +91,7 @@ Go conventions to follow:
 - Always handle errors; don't ignore return values
 - **Migrations:** When adding a new SQL migration file in `migrations/`, bump `RequiredSchemaVersion` in `internal/upgrade/version.go` to match the new migration number
 - **i18n strings:** When adding user-facing error messages, add key to `internal/i18n/keys.go` and translations to `catalog_en.go`, `catalog_vi.go`, `catalog_zh.go`. For UI strings, add to all locale JSON files in `ui/web/src/i18n/locales/{en,vi,zh}/`
+- **SQL safety:** When implementing or modifying SQL store code (`store/pg/*.go`), always verify: (1) All user inputs use parameterized queries (`$1, $2, ...`), never string concatenation — prevents SQL injection. (2) Queries are optimized — no N+1 queries, no unnecessary full table scans. (3) WHERE clauses, JOINs, and ORDER BY columns use existing indices — check migration files for available indexes
 
 ## Mobile UI/UX Rules