feat: add bounded postgres data migration stack

docs/postgres-migration.md

@@ -0,0 +1,23 @@
+# PostgreSQL migration
+
+The Compose stack includes PostgreSQL with a hard 512 MB container limit and
+conservative in-server memory settings. Set `POSTGRES_PASSWORD` in the server
+`.env`, then start it with `docker compose up -d postgres`.
+
+Run the resumable logical copy with:
+
+```sh
+docker compose --profile migration run --rm postgres-migrate
+```
+
+It copies archive data to the `archive` schema and autonomy/intelligence data
+to the `intelligence` schema. Each table is count-verified and progress is
+persisted in each schema's `_migration_progress` table, so restarting resumes
+at the last committed batch. SQLite vec0 internal tables are derived indexes;
+their canonical embedding data is migrated from `article_embedding_store`.
+
+The application is intentionally not switched by this migration alone: it has
+SQLite-specific synchronous query and transaction code in the autonomy workers
+and admin routes. Pointing those processes at PostgreSQL before that code is
+ported would make the live system unreliable. Keep SQLite mounted until a
+separate verified runtime cutover is complete.