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/pact/capabilities.go

@@ -686,6 +686,15 @@ type AdminPartialData interface {
 // set, turns a stored value into the value the element receives. ListValue,
 // when set, returns the text a list column of this type shows. Upload, when
 // set, stores a file the element uploads and returns its URL.
+//
+// RequestOptions, when set, adds options that change per request, such as a
+// site setting the element follows. The admin calls it once per form schema
+// request for each type the form uses, with the application's database (nil
+// when none is configured), and merges the answer into every field's
+// options: an option the field sets in fields.yaml wins. An error is logged
+// and the schema is served with the fields.yaml options only. The options
+// reach every administrator who may open the form, so they must hold no
+// secret.
 type FormFieldType struct {
 	Name         string
 	Element      string
@@ -698,6 +707,9 @@ type FormFieldType struct {
 	Load         func(ctx context.Context, in FormFieldValue) (any, error)                    `json:"-"`
 	ListValue    func(ctx context.Context, value any) string                                  `json:"-"`
 	Upload       func(ctx context.Context, in FormFieldUpload) (FormFieldUploadResult, error) `json:"-"`
+	// RequestOptions answers options for one form schema request; nil when
+	// the type has none.
+	RequestOptions func(ctx context.Context, db *gorm.DB) (map[string]any, error) `json:"-"`
 }
 
 // FormFieldValue is what the framework hands Decode and Load. DB is the save