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/__snapshots__/test_cli/test_schema_cache_snapshot[dbTables].yaml

@@ -8,6 +8,7 @@
     tableName: authors_only
     tablePKCols: []
     tableSchema: public
+    tableUniqueCols: []
     tableUpdatable: true
 
 - - qiName: cats
@@ -39,6 +40,7 @@
     tablePKCols:
     - id
     tableSchema: public
+    tableUniqueCols: []
     tableUpdatable: true
 
 - - qiName: items_w_isolation_level
@@ -69,6 +71,7 @@
     tableName: items_w_isolation_level
     tablePKCols: []
     tableSchema: public
+    tableUniqueCols: []
     tableUpdatable: true
 
 - - qiName: directors
@@ -100,6 +103,7 @@
     tablePKCols:
     - id
     tableSchema: public
+    tableUniqueCols: []
     tableUpdatable: true
 
 - - qiName: projects
@@ -112,6 +116,7 @@
     tableName: projects
     tablePKCols: []
     tableSchema: public
+    tableUniqueCols: []
     tableUpdatable: true
 
 - - qiName: infinite_recursion
@@ -124,6 +129,7 @@
     tableName: infinite_recursion
     tablePKCols: []
     tableSchema: public
+    tableUniqueCols: []
     tableUpdatable: false
 
 - - qiName: awards
@@ -182,6 +188,7 @@
     tablePKCols:
     - id
     tableSchema: public
+    tableUniqueCols: []
     tableUpdatable: true
 
 - - qiName: films
@@ -222,6 +229,7 @@
     tablePKCols:
     - id
     tableSchema: public
+    tableUniqueCols: []
     tableUpdatable: true
 
 - - qiName: items
@@ -243,4 +251,5 @@
     tableName: items
     tablePKCols: []
     tableSchema: public
+    tableUniqueCols: []
     tableUpdatable: true