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/migrations/script.py.mako

@@ -0,0 +1,26 @@
+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+${imports if imports else ""}
+
+# revision identifiers, used by Alembic.
+revision: str = ${repr(up_revision)}
+down_revision: Union[str, None] = ${repr(down_revision)}
+branch_labels: Union[str, Sequence[str], None] = ${repr(branch_labels)}
+depends_on: Union[str, Sequence[str], None] = ${repr(depends_on)}
+
+
+def upgrade() -> None:
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade() -> None:
+    ${downgrades if downgrades else "pass"}