Initial fullstack project setup with Next.js 15, Gin, PostgreSQL and Docker Compose

- Frontend: Next.js 15 (App Router), Auth.js v5, shadcn/ui, MagicUI
- Backend: Go + Gin + GORM with layered architecture
- Auth: Local credentials login with optional Keycloak OAuth binding
- Admin: RBAC user management for admin role
- Dev: Docker Compose with hot reload for both frontend and backend
- Docker: 3-service orchestration (frontend, backend, postgres)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/domain/user.go

@@ -0,0 +1,15 @@
+package domain
+
+import "time"
+
+type User struct {
+	ID            uint      `gorm:"primarykey" json:"id"`
+	Username      string    `gorm:"uniqueIndex;not null" json:"username"`
+	Email         string    `gorm:"uniqueIndex;not null" json:"email"`
+	PasswordHash  string    `gorm:"not null" json:"-"`
+	Role          string    `gorm:"default:'user';not null" json:"role"`
+	KeycloakID    *string   `gorm:"uniqueIndex" json:"keycloakId,omitempty"`
+	KeycloakEmail string    `json:"keycloakEmail,omitempty"`
+	CreatedAt     time.Time `json:"createdAt"`
+	UpdatedAt     time.Time `json:"updatedAt"`
+}