feat(server): add brand kit management and project binding

Add a brand kit domain with memory and postgres stores, a BrandKitService
for CRUD and resolution, and REST endpoints to list, upsert, and delete
brand kits. Projects can bind a brand kit whose resolved context and
reference images feed into the agent's long-term memory and design prompts.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/domain/brandkit/brand_kit.go

@@ -0,0 +1,38 @@
+package brandkit
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"time"
+)
+
+var (
+	ErrInvalidInput = errors.New("invalid brand kit")
+	ErrNotFound     = errors.New("brand kit not found")
+	ErrConflict     = errors.New("brand kit conflict")
+)
+
+type Kit struct {
+	ID        string
+	UserID    string
+	Name      string
+	Document  json.RawMessage
+	IsDefault bool
+	CreatedAt time.Time
+	UpdatedAt time.Time
+}
+
+type Store interface {
+	List(ctx context.Context, userID string) ([]Kit, error)
+	Get(ctx context.Context, userID string, id string) (Kit, error)
+	GetDefault(ctx context.Context, userID string) (Kit, error)
+	Upsert(ctx context.Context, kit Kit) (Kit, error)
+	Delete(ctx context.Context, userID string, id string) error
+	Close() error
+}
+
+func Clone(kit Kit) Kit {
+	kit.Document = append(json.RawMessage(nil), kit.Document...)
+	return kit
+}

server/internal/domain/design/project.go

@@ -201,6 +201,9 @@ type Project struct {
 	SnapshotID   string
 	Canvas       string
 	LastThreadID string
+	BrandKitID   string
+	BrandContext string
+	BrandImages  []string
 	Viewport     Viewport
 	Nodes        []Node
 	Connections  []Connection
@@ -208,13 +211,14 @@ type Project struct {
 }
 
 type ProjectSummary struct {
-	ID        string
-	UserID    string
-	Title     string
-	Brief     string
-	Status    ProjectStatus
-	Thumbnail string
-	UpdatedAt time.Time
+	ID         string
+	UserID     string
+	Title      string
+	Brief      string
+	Status     ProjectStatus
+	Thumbnail  string
+	BrandKitID string
+	UpdatedAt  time.Time
 }
 
 type QuickAction struct {