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/infrastructure/piagent/creative_agent.go

@@ -843,6 +843,11 @@ func projectBriefContext(project design.Project) string {
 		builder.WriteString(project.Brief)
 		builder.WriteString("\n")
 	}
+	if strings.TrimSpace(project.BrandContext) != "" {
+		builder.WriteString("Brand Kit context:\n")
+		builder.WriteString(strings.TrimSpace(project.BrandContext))
+		builder.WriteString("\n")
+	}
 	builder.WriteString(fmt.Sprintf("Canvas nodes: %d", len(project.Nodes)))
 	return builder.String()
 }