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/spec/fixtures/schema.sql

@@ -1451,6 +1451,15 @@ create table test.compound_unique(
   unique(key1, key2)
 );
 
+create table test.mixed_unique(
+  id integer not null,
+  key1 integer not null,
+  key2 integer not null,
+  value text,
+  unique(id),
+  unique(key1, key2)
+);
+
 create table test.family_tree (
   id text not null primary key,
   name text not null,