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/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 {