feat(cabana): per-request options for plugin field types

- pact.FormFieldType gains RequestOptions, called with the application's
  database once per form schema request for each type the form uses
- the form schema route merges the answer into a copy of each field's
  typeOptions; a fields.yaml key wins, the compiled schema never changes
- an error logs one warning naming the type and serves the fields.yaml
  options only
- TestFieldTypeRequestOptions covers every screen, the precedence, the
  call count, the error path, unchanged schemas, form extension fields
  and the authenticated handler; a runnable example backs the docs
- the typeOptions description, the regenerated OpenAPI document and
  schema.d.ts, the field types page and the pact and cabana READMEs
  document it

modules/cabana/http.go

@@ -857,6 +857,9 @@ func (s *service) formSchema(w http.ResponseWriter, r *http.Request) {
 			WriteError(w, http.StatusInternalServerError, "error", msgServerError)
 			return
 		}
+		// A plugin field type's per-request options (pact.FormFieldType.
+		// RequestOptions) join its fields.yaml options in kept only.
+		mergeRequestOptions(r.Context(), s.requestDB(r.Context()), kept)
 		view.Fields = kept
 		view.Assets = s.controllerAssets(cc)
 		if codes := s.enabledContentLocales(r.Context()); len(codes) > 0 {
@@ -873,6 +876,16 @@ func (s *service) formSchema(w http.ResponseWriter, r *http.Request) {
 	})
 }
 
+// requestDB is the application's database bound to ctx, nil when none is
+// configured.
+func (s *service) requestDB(ctx context.Context) *gorm.DB {
+	db, err := s.db()
+	if err != nil {
+		return nil
+	}
+	return db.WithContext(ctx)
+}
+
 // schemaFieldHider is implemented by cabana's own admin controllers that
 // hide some fields from some administrators (the Administrators screen's
 // is_superuser for non-superusers). hiddenSchemaFields names the form