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>

migrations/versions/2025_01_16_0100-d4e5f6g7h8i9_add_user_auth.py

@@ -0,0 +1,42 @@
+"""Add user authentication table.
+
+Revision ID: d4e5f6g7h8i9
+Revises: c3d4e5f6g7h8
+Create Date: 2025-01-16 01:00:00.000000
+
+"""
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision = "d4e5f6g7h8i9"
+down_revision = "c3d4e5f6g7h8"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "user",
+        sa.Column("id", sa.Integer(), nullable=False),
+        sa.Column("username", sa.String(length=64), nullable=False),
+        sa.Column("hashed_password", sa.String(length=128), nullable=False),
+        sa.Column(
+            "role",
+            sa.String(length=16),
+            nullable=False,
+            server_default="viewer",
+            comment="Role: 'admin', 'editor', 'operator', 'viewer'.",
+        ),
+        sa.Column("is_active", sa.Boolean(), nullable=False, server_default=sa.text("1")),
+        sa.Column("created_at", sa.DateTime(), nullable=False),
+        sa.PrimaryKeyConstraint("id"),
+    )
+    op.create_index(op.f("ix_user_id"), "user", ["id"], unique=False)
+    op.create_index(op.f("ix_user_username"), "user", ["username"], unique=True)
+
+
+def downgrade() -> None:
+    op.drop_index(op.f("ix_user_username"), table_name="user")
+    op.drop_index(op.f("ix_user_id"), table_name="user")
+    op.drop_table("user")