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/package.json

@@ -1,6 +1,6 @@
 {
   "name": "hoa-ledgeriq-frontend",
-  "version": "2026.03.16",
+  "version": "2026.3.17",
   "private": true,
   "type": "module",
   "scripts": {
@@ -16,6 +16,7 @@
     "@mantine/hooks": "^7.15.3",
     "@mantine/modals": "^7.15.3",
     "@mantine/notifications": "^7.15.3",
+    "@simplewebauthn/browser": "^13.3.0",
     "@tabler/icons-react": "^3.28.1",
     "@tanstack/react-query": "^5.64.2",
     "axios": "^1.7.9",