feat: expose unique columns and many-to-many markers in OpenAPI

Add unique constraint and many-to-many relationship metadata to the
generated OpenAPI spec so clients can render them.

- Store unique constraints on Table as tableUniqueCols (mirroring
  tablePKCols) instead of denormalizing them onto each Column.
- Compute unique constraints via a per-table tbl_unique_cols CTE in
  tablesSqlQuery.
- Annotate unique columns and composite unique constraints in property
  descriptions, and emit m2m markers in table descriptions.

test/io/test_connection.py

@@ -239,7 +239,7 @@ def test_pool_acquisition_timeout(level, defaultenv, metapostgrest):
         assert data["message"] == "Timed out acquiring connection from connection pool."
 
         # ensure the message appears on the logs as well
-        output = sorted(postgrest.read_stdout(nlines=10))
+        output = sorted(drain_stdout(postgrest))
 
         if level == "crit":
             assert len(output) == 0