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/database/models.py

@@ -163,3 +163,20 @@ class SpoolField(Base):
     spool: Mapped["Spool"] = relationship(back_populates="extra")
     key: Mapped[str] = mapped_column(String(64), primary_key=True, index=True)
     value: Mapped[str] = mapped_column(Text())
+
+
+class User(Base):
+    """User account for authentication."""
+
+    __tablename__ = "user"
+
+    id: Mapped[int] = mapped_column(primary_key=True, index=True)
+    username: Mapped[str] = mapped_column(String(64), unique=True, index=True)
+    hashed_password: Mapped[str] = mapped_column(String(128))
+    role: Mapped[str] = mapped_column(
+        String(16),
+        default="viewer",
+        comment="Role: 'admin', 'editor', 'operator', 'viewer'.",
+    )
+    is_active: Mapped[bool] = mapped_column(default=True)
+    created_at: Mapped[datetime] = mapped_column()