feat(10-02): filter choices and a fully typed admin API proven on the wire

- pact.FilterOptions on the model serves a scope filter's choices; a scope
  filter whose model lacks it fails activation (D-27)
- GET /{vendor}/{plugin}/{controller}/filters/{scope}/options answers a
  declared scope filter behind the controller permission with localized
  {value, label} choices, 404 otherwise
- Every admin route documents a typed success schema, and protected routes
  document 401, 403 and 404 (422 on writes); SuccessEnvelope is gone and
  logout writes a typed AdminLogoutData
- jsonScalar and fieldContext decode their served shapes
- TestPhase10OpenAPIConformance calls every inventoried route through the
  assembled router on PostgreSQL and decodes each body into its documented
  type with unknown fields disallowed, checking admin.json's schema ref
- The SPA aliases every new schema type; Tailwind no longer scans the
  generated API files, so API changes do not churn boardwalk/dist

cabana/schema_types.go

@@ -3,6 +3,7 @@ package cabana
 import (
 	"context"
 	"encoding/json"
+	"fmt"
 )
 
 // ListSort is the compiled default sort. Direction is exactly asc or desc.
@@ -191,12 +192,45 @@ func (s jsonScalar) MarshalJSON() ([]byte, error) {
 	return s.raw, nil
 }
 
+// UnmarshalJSON accepts exactly a JSON scalar (string, number, boolean or
+// null), so a served schema decodes back into its documented type.
+func (s *jsonScalar) UnmarshalJSON(raw []byte) error {
+	var value any
+	if err := json.Unmarshal(raw, &value); err != nil {
+		return err
+	}
+	switch value.(type) {
+	case nil:
+		s.raw = nil
+	case string, float64, bool:
+		s.raw = append(json.RawMessage(nil), raw...)
+	default:
+		return fmt.Errorf("cabana: %s is not a JSON scalar", raw)
+	}
+	return nil
+}
+
 // fieldContext preserves a single Winter context string or a source-ordered list.
 type fieldContext struct {
 	single bool
 	values []string
 }
 
+// UnmarshalJSON accepts the two served shapes: a string or a string list.
+func (c *fieldContext) UnmarshalJSON(raw []byte) error {
+	var single string
+	if err := json.Unmarshal(raw, &single); err == nil {
+		*c = fieldContext{single: true, values: []string{single}}
+		return nil
+	}
+	var values []string
+	if err := json.Unmarshal(raw, &values); err != nil {
+		return fmt.Errorf("cabana: context must be a string or a list of strings")
+	}
+	*c = fieldContext{values: values}
+	return nil
+}
+
 func (c fieldContext) MarshalJSON() ([]byte, error) {
 	if c.single {
 		text := ""