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>

backend/app/models/department.py

@@ -0,0 +1,17 @@
+import uuid
+from sqlalchemy import Column, String, ForeignKey, DateTime
+from sqlalchemy.sql import func
+from sqlalchemy.orm import relationship
+from app.core.database import Base
+
+
+class Department(Base):
+    __tablename__ = "pjctrl_departments"
+
+    id = Column(String(36), primary_key=True, default=lambda: str(uuid.uuid4()))
+    name = Column(String(100), nullable=False)
+    parent_id = Column(String(36), ForeignKey("pjctrl_departments.id"), nullable=True)
+    created_at = Column(DateTime, server_default=func.now())
+
+    # Self-referential relationship
+    parent = relationship("Department", remote_side=[id], backref="children")