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/Feature/OpenApi/OpenApiSpec.hs

@@ -285,10 +285,58 @@ spec withConfig = withConfig baseCfg $ describe "OpenAPI" $ do
             {
               "format": "int32",
               "type": "integer",
-              "description": "Note:\nThis is a Foreign Key to `second.id`.<fk table='second' column='id'/>"
+              "description": "Note:\nThis is a Unique column.<unique/>\nThis is a Foreign Key to `second.id`.<fk table='second' column='id'/>"
             }
           |]
 
+    it "includes a unique description for a column with a unique constraint" $ do
+      r <- simpleBody <$> get "/"
+
+      let uniqueKey = r ^? key "definitions" . key "single_unique" . key "properties" . key "unique_key"
+
+      liftIO $
+        uniqueKey `shouldBe` Just
+          [aesonQQ|
+            {
+              "format": "int32",
+              "type": "integer",
+              "description": "Note:\nThis is a Unique column.<unique/>"
+            }
+          |]
+
+    it "includes the column list of a composite unique constraint" $ do
+      r <- simpleBody <$> get "/"
+
+      let compoundKey1 = r ^? key "definitions" . key "compound_unique" . key "properties" . key "key1"
+          compoundKey2 = r ^? key "definitions" . key "compound_unique" . key "properties" . key "key2"
+
+      liftIO $ do
+        compoundKey1 `shouldBe` Just
+          [aesonQQ|
+            {
+              "format": "int32",
+              "type": "integer",
+              "description": "Note:\nThis is part of a composite unique constraint.<unique cols='key1,key2'/>"
+            }
+          |]
+        compoundKey2 `shouldBe` Just
+          [aesonQQ|
+            {
+              "format": "int32",
+              "type": "integer",
+              "description": "Note:\nThis is part of a composite unique constraint.<unique cols='key1,key2'/>"
+            }
+          |]
+
+    it "includes m2m relationship markers in the table description" $ do
+      r <- simpleBody <$> get "/"
+
+      let beingDescription = r ^? key "definitions" . key "being" . key "description"
+
+      liftIO $
+        beingDescription `shouldBe` Just
+          [aesonQQ|"<m2m table='part' junction='being_part' source='being' target='part'/>"|]
+
   describe "Foreign table" $
 
     it "includes foreign table properties" $ do