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>

client/src/pages/login/index.tsx

@@ -0,0 +1,98 @@
+import { LockOutlined, UserOutlined } from "@ant-design/icons";
+import { Button, Card, Form, Input, message, Typography } from "antd";
+import React, { useState } from "react";
+import { useNavigate } from "react-router";
+import { useAuth } from "../../contexts/auth";
+
+const { Title, Text } = Typography;
+
+interface LoginForm {
+  username: string;
+  password: string;
+}
+
+export const LoginPage: React.FC = () => {
+  const { login, setup, authStatus, refreshAuthStatus } = useAuth();
+  const navigate = useNavigate();
+  const [isLoading, setIsLoading] = useState(false);
+  const [isSetupMode, setIsSetupMode] = useState(false);
+
+  const needsSetup = authStatus?.enabled && !authStatus?.has_users;
+
+  const handleSubmit = async (values: LoginForm) => {
+    setIsLoading(true);
+    try {
+      if (needsSetup || isSetupMode) {
+        await setup(values.username, values.password);
+        message.success("Admin account created successfully!");
+      } else {
+        await login(values.username, values.password);
+        message.success("Logged in successfully!");
+      }
+      navigate("/");
+    } catch (err) {
+      message.error(err instanceof Error ? err.message : "Authentication failed");
+    } finally {
+      setIsLoading(false);
+    }
+  };
+
+  return (
+    <div
+      style={{
+        display: "flex",
+        justifyContent: "center",
+        alignItems: "center",
+        minHeight: "100vh",
+        background: "#f0f2f5",
+      }}
+    >
+      <Card style={{ width: 400, boxShadow: "0 4px 12px rgba(0,0,0,0.1)" }}>
+        <div style={{ textAlign: "center", marginBottom: 24 }}>
+          <Title level={2} style={{ marginBottom: 8 }}>
+            Spoolman
+          </Title>
+          <Text type="secondary">
+            {needsSetup || isSetupMode ? "Create Admin Account" : "Sign in to continue"}
+          </Text>
+        </div>
+
+        <Form<LoginForm> name="login" onFinish={handleSubmit} layout="vertical" requiredMark={false}>
+          <Form.Item
+            name="username"
+            rules={[
+              { required: true, message: "Please enter your username" },
+              { min: 3, message: "Username must be at least 3 characters" },
+            ]}
+          >
+            <Input prefix={<UserOutlined />} placeholder="Username" size="large" autoFocus />
+          </Form.Item>
+
+          <Form.Item
+            name="password"
+            rules={[
+              { required: true, message: "Please enter your password" },
+              { min: 6, message: "Password must be at least 6 characters" },
+            ]}
+          >
+            <Input.Password prefix={<LockOutlined />} placeholder="Password" size="large" />
+          </Form.Item>
+
+          <Form.Item style={{ marginBottom: 12 }}>
+            <Button type="primary" htmlType="submit" size="large" block loading={isLoading}>
+              {needsSetup || isSetupMode ? "Create Account" : "Sign In"}
+            </Button>
+          </Form.Item>
+        </Form>
+
+        {needsSetup && (
+          <Text type="secondary" style={{ display: "block", textAlign: "center" }}>
+            This is the first time setup. Create an admin account to get started.
+          </Text>
+        )}
+      </Card>
+    </div>
+  );
+};
+
+export default LoginPage;