feat: Add complete user system implementation

- Create User, Role, Permission entities with their relationships
- Implement service interfaces and implementations for user system
- Add controllers for User, Role, Permission, UserRole, and RolePermission
- Include SQL schema files for the user system
- Document project structure in CLAUDE.md

This adds a complete RBAC (Role-Based Access Control) system with:
- User entity for managing user accounts
- Role entity for defining roles
- Permission entity for defining permissions
- UserRole entity for user-role relationships
- RolePermission entity for role-permission relationships

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.claude/settings.local.json

@@ -0,0 +1,7 @@
+{
+  "permissions": {
+    "allow": [
+      "Bash(git add:*)"
+    ]
+  }
+}