chore(db): clean legacy sql assets and standardize migrations

database/README.md

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+# SQL Directory Guide
+
+This project uses Flyway as the single source of truth for schema changes.
+
+## Active SQL
+
+- `src/main/resources/db/migration/*.sql`: production migrations (required)
+- `database/init/`: MySQL first-init directory (keep minimal)
+
+## Optional reference SQL
+
+- `database/flowable/`: Flowable engine schema scripts for multiple databases
+- `database/upgrade/`: BladeX cross-database upgrade reference scripts
+
+## Cleanup rule
+
+- Do not place full database dumps under `database/` root.
+- Keep backups outside repository (off-repo storage or backup system).