feat(auth): Add optional authentication system (#22)

Backend:
- Add User model with username, hashed password, role, is_active
- Add passlib[bcrypt] and python-jose[cryptography] dependencies
- Create auth.py with JWT utilities, password hashing, role checking
- Add /auth endpoints: status, setup, login, me, users CRUD
- Add role-based permission system (viewer/operator/editor/admin)
- Environment: SPOOLMAN_AUTH_ENABLED, SPOOLMAN_AUTH_SECRET_KEY
- Migration for user table

Frontend:
- Add AuthProvider context with login/logout/setup
- Add LoginPage component with setup mode for first user
- Add ProtectedRoute wrapper for auth-required pages
- Add axios interceptor to attach JWT token to requests
- Add User Management tab in Settings (admin only)
- Add translation keys for auth UI

When SPOOLMAN_AUTH_ENABLED=true:
- Users must login to access the app
- First visit shows setup page to create admin account
- Role-based access control for future endpoint protection

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

spoolman/api/v1/router.py

@@ -15,7 +15,7 @@ from spoolman.database.database import backup_global_db
 from spoolman.exceptions import ItemNotFoundError
 from spoolman.ws import websocket_manager
 
-from . import backup as backup_module, export, externaldb, field, filament, models, other, print_job, setting, spool, vendor
+from . import auth, backup as backup_module, export, externaldb, field, filament, models, other, print_job, setting, spool, vendor
 
 logger = logging.getLogger(__name__)
 
@@ -57,6 +57,7 @@ async def info() -> models.Info:
         db_type=str(env.get_database_type() or "sqlite"),
         git_commit=env.get_commit_hash(),
         build_date=env.get_build_date(),
+        auth_enabled=auth.is_auth_enabled(),
     )
 
 
@@ -114,3 +115,4 @@ app.include_router(externaldb.router)
 app.include_router(export.router)
 app.include_router(backup_module.router)
 app.include_router(print_job.router)
+app.include_router(auth.router)