chore: update docs

backend/src/_system/CLAUDE.md

@@ -0,0 +1,10 @@
+# _system/
+
+## Subdirectories
+
+| Directory | What | When to read |
+| --------- | ---- | ------------ |
+| `cli/` | CLI commands and tools | Running backend CLI commands |
+| `migrations/` | Database migration runner | Running or writing migrations |
+| `schema/` | Database schema generation | Schema export, documentation |
+| `scripts/` | System utility scripts | Database maintenance, automation |