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_log.py

@@ -44,7 +44,7 @@ def test_log_level(level, defaultenv):
         response = postgrest.session.get("/")
         assert response.status_code == 200
 
-        output = postgrest.read_stdout(nlines=9)
+        output = drain_stdout(postgrest)
 
         if level == "crit":
             assert len(output) == 0
@@ -82,7 +82,7 @@ def test_log_level(level, defaultenv):
                     r'- - postgrest_test_anonymous \[.+\] "GET / HTTP/1.1" 200 \d+ "" "python-requests/.+"',
                 ],
             )
-            assert len(output) == 9
+            assert len(output) > 3
             assert any("Connection" and "is available" in line for line in output)
             assert any("Connection" and "is used" in line for line in output)
 
@@ -403,7 +403,7 @@ def test_db_error_logging_to_stderr(level, defaultenv, metapostgrest):
         assert response.status_code == 500
 
         # ensure the message appears on the logs
-        output = postgrest.read_stdout(nlines=8)
+        output = drain_stdout(postgrest)
 
         if level == "crit":
             assert len(output) == 0