docs: reorder & testing

backend/internal/postgres/migrations/README.md

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+# Backend migrations
+
+Goose migrations embedded into the backend binary by `embed.go`. Applied
+at startup before any listener opens (see `internal/postgres`).
+
+## Pre-production single-file rule
+
+**While the platform is not yet in production, every schema change goes
+into the existing `00001_init.sql` file** rather than a new
+`00002_*`-prefixed file. The intent is to keep the schema in one
+canonical place so reviewers and developers do not have to reconstruct
+the latest shape from a chain of incremental migrations.
+
+Operationally this means that pulling a branch with schema changes
+requires a fresh database — the only consumer today is local development
+and integration tests, both of which spin up disposable Postgres
+instances.
+
+> **Remove this rule before the first production deployment.** From
+> that point on every schema change must be a new migration file with a
+> monotonically increasing prefix, and `00001_init.sql` becomes
+> immutable history.
+
+If you need to make a change, edit `00001_init.sql` directly. Down
+migrations should still be kept in sync (they live at the bottom of the
+file — currently a single `DROP SCHEMA backend CASCADE`).