chore: implement NPM/JSON import routes and fix SMTP persistence

Phase 3 of skipped tests remediation - enables 7 previously skipped E2E tests

Backend:

Add NPM import handler with session-based upload/commit/cancel
Add JSON import handler with Charon/NPM format support
Fix SMTP SaveSMTPConfig using transaction-based upsert
Add comprehensive unit tests for new handlers
Frontend:

Add ImportNPM page component following ImportCaddy pattern
Add ImportJSON page component with format detection
Add useNPMImport and useJSONImport React Query hooks
Add API clients for npm/json import endpoints
Register routes in App.tsx and navigation in Layout.tsx
Add i18n keys for new import pages
Tests:

7 E2E tests now enabled and passing
Backend coverage: 86.8%
Reduced total skipped tests from 98 to 91
Closes: Phase 3 of skipped-tests-remediation plan

frontend/src/components/Layout.tsx

@@ -100,6 +100,8 @@ export default function Layout({ children }: LayoutProps) {
           children: [
             { name: t('navigation.caddyfile'), path: '/tasks/import/caddyfile', icon: '📥' },
             { name: t('navigation.crowdsec'), path: '/tasks/import/crowdsec', icon: '🛡️' },
+            { name: t('navigation.importNPM'), path: '/tasks/import/npm', icon: '📦' },
+            { name: t('navigation.importJSON'), path: '/tasks/import/json', icon: '📄' },
           ]
         },
         { name: t('navigation.backups'), path: '/tasks/backups', icon: '💾' },