feat: implement user authentication module

- Backend (FastAPI):
  - External API authentication (pj-auth-api.vercel.app)
  - JWT token validation with Redis session storage
  - RBAC with department isolation
  - User, Role, Department models with pjctrl_ prefix
  - Alembic migrations with project-specific version table
  - Complete test coverage (13 tests)

- Frontend (React + Vite):
  - AuthContext for state management
  - Login page with error handling
  - Protected route component
  - Dashboard with user info display

- OpenSpec:
  - 7 capability specs defined
  - add-user-auth change archived

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontend/src/components/ProtectedRoute.tsx

@@ -0,0 +1,21 @@
+import { Navigate } from 'react-router-dom'
+import { useAuth } from '../contexts/AuthContext'
+import { ReactNode } from 'react'
+
+interface ProtectedRouteProps {
+  children: ReactNode
+}
+
+export default function ProtectedRoute({ children }: ProtectedRouteProps) {
+  const { isAuthenticated, loading } = useAuth()
+
+  if (loading) {
+    return <div className="container">Loading...</div>
+  }
+
+  if (!isAuthenticated) {
+    return <Navigate to="/login" />
+  }
+
+  return <>{children}</>
+}