Add user authentication system with API key management (#50)

- Add User, Session, AuthSettings models with bcrypt password hashing
- Add auth endpoints: login, logout, change-password, me
- Add API key CRUD: create (orch_xxx format), list, revoke
- Add admin user management: list, create, update, reset-password
- Create default admin user on startup (admin/admin)
- Add frontend: Login page, API Keys page, Admin Users page
- Add AuthContext for session state management
- Add user menu to Layout header with login/logout/settings
- Add 15 integration tests for auth system
- Add migration 006_auth_tables.sql

frontend/src/contexts/AuthContext.tsx

@@ -0,0 +1,87 @@
+import { createContext, useContext, useState, useEffect, useCallback, ReactNode } from 'react';
+import { User } from '../types';
+import { getCurrentUser, login as apiLogin, logout as apiLogout } from '../api';
+
+interface AuthContextType {
+  user: User | null;
+  loading: boolean;
+  error: string | null;
+  login: (username: string, password: string) => Promise<void>;
+  logout: () => Promise<void>;
+  clearError: () => void;
+}
+
+const AuthContext = createContext<AuthContextType | undefined>(undefined);
+
+interface AuthProviderProps {
+  children: ReactNode;
+}
+
+export function AuthProvider({ children }: AuthProviderProps) {
+  const [user, setUser] = useState<User | null>(null);
+  const [loading, setLoading] = useState(true);
+  const [error, setError] = useState<string | null>(null);
+
+  // Check session on initial load
+  useEffect(() => {
+    async function checkAuth() {
+      try {
+        const currentUser = await getCurrentUser();
+        setUser(currentUser);
+      } catch {
+        setUser(null);
+      } finally {
+        setLoading(false);
+      }
+    }
+    checkAuth();
+  }, []);
+
+  const login = useCallback(async (username: string, password: string) => {
+    setLoading(true);
+    setError(null);
+    try {
+      const loggedInUser = await apiLogin({ username, password });
+      setUser(loggedInUser);
+    } catch (err) {
+      const message = err instanceof Error ? err.message : 'Login failed';
+      setError(message);
+      throw err;
+    } finally {
+      setLoading(false);
+    }
+  }, []);
+
+  const logout = useCallback(async () => {
+    setLoading(true);
+    setError(null);
+    try {
+      await apiLogout();
+      setUser(null);
+    } catch (err) {
+      const message = err instanceof Error ? err.message : 'Logout failed';
+      setError(message);
+      throw err;
+    } finally {
+      setLoading(false);
+    }
+  }, []);
+
+  const clearError = useCallback(() => {
+    setError(null);
+  }, []);
+
+  return (
+    <AuthContext.Provider value={{ user, loading, error, login, logout, clearError }}>
+      {children}
+    </AuthContext.Provider>
+  );
+}
+
+export function useAuth() {
+  const context = useContext(AuthContext);
+  if (context === undefined) {
+    throw new Error('useAuth must be used within an AuthProvider');
+  }
+  return context;
+}