feat: SaaS onboarding, Stripe billing, MFA, SSO, passkeys, refresh tokens

Complete SaaS self-service onboarding sprint:

- Stripe-powered signup flow: pricing page → checkout → provisioning → activation
- Refresh token infrastructure: 1h access tokens + 30-day httpOnly cookie refresh
- TOTP MFA with QR setup, recovery codes, and login challenge flow
- Google + Azure AD SSO (conditional on env vars) with account linking
- WebAuthn passkey registration and passwordless login
- Guided onboarding checklist with server-side progress tracking
- Stubbed email service (console + DB logging, ready for real provider)
- Settings page with tabbed security settings (MFA, passkeys, linked accounts)
- Login page enhanced with MFA verification, SSO buttons, passkey login
- Database migration 015 with all new tables and columns
- Version bump to 2026.03.17

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/onboarding/OnboardingPendingPage.tsx

@@ -0,0 +1,82 @@
+import { useEffect, useState } from 'react';
+import { Container, Center, Stack, Loader, Text, Title, Alert, Button } from '@mantine/core';
+import { IconCheck, IconAlertCircle } from '@tabler/icons-react';
+import { useSearchParams, useNavigate } from 'react-router-dom';
+import api from '../../services/api';
+
+export function OnboardingPendingPage() {
+  const [searchParams] = useSearchParams();
+  const navigate = useNavigate();
+  const sessionId = searchParams.get('session_id');
+  const [status, setStatus] = useState<string>('polling');
+  const [error, setError] = useState('');
+
+  useEffect(() => {
+    if (!sessionId) {
+      setError('No session ID provided');
+      return;
+    }
+
+    let cancelled = false;
+    const poll = async () => {
+      try {
+        const { data } = await api.get(`/billing/status?session_id=${sessionId}`);
+        if (cancelled) return;
+
+        if (data.status === 'active') {
+          setStatus('complete');
+          // Redirect to login page — user will get activation email
+          setTimeout(() => navigate('/login'), 3000);
+        } else if (data.status === 'not_configured') {
+          setError('Payment system is not configured. Please contact support.');
+        } else {
+          // Still provisioning — poll again
+          setTimeout(poll, 3000);
+        }
+      } catch (err: any) {
+        if (!cancelled) {
+          setError(err.response?.data?.message || 'Failed to check status');
+        }
+      }
+    };
+
+    poll();
+    return () => { cancelled = true; };
+  }, [sessionId, navigate]);
+
+  return (
+    <Container size="sm" py={80}>
+      <Center>
+        <Stack align="center" gap="lg">
+          {error ? (
+            <>
+              <Alert icon={<IconAlertCircle size={16} />} color="red" variant="light">
+                {error}
+              </Alert>
+              <Button variant="light" onClick={() => navigate('/pricing')}>
+                Back to Pricing
+              </Button>
+            </>
+          ) : status === 'complete' ? (
+            <>
+              <IconCheck size={48} color="var(--mantine-color-green-6)" />
+              <Title order={2}>Your account is ready!</Title>
+              <Text c="dimmed" ta="center">
+                Check your email for an activation link to set your password and get started.
+              </Text>
+              <Text size="sm" c="dimmed">Redirecting to login...</Text>
+            </>
+          ) : (
+            <>
+              <Loader size="xl" />
+              <Title order={2}>Setting up your account...</Title>
+              <Text c="dimmed" ta="center" maw={400}>
+                We're creating your HOA workspace. This usually takes just a few seconds.
+              </Text>
+            </>
+          )}
+        </Stack>
+      </Center>
+    </Container>
+  );
+}