feat(phase-9): email dispatcher — HTML templates, delivery log, test endpoint

Backend:
- app/models/email_log.py: EmailLog table (school_id, to, subject, type, status,
  error, sent_at) with EmailType + EmailStatus enums
- migrations/002_phase9_email_logs.py: Alembic migration for email_logs table
- app/templates/email/: 6 Jinja2 HTML templates — base layout, invoice,
  low_credit, license_expiry, overdue_warning, suspension
- app/services/email.py: enhanced send_email() — accepts template_name+context
  for HTML rendering, logs every attempt to email_logs, retries up to 3x on
  transient SMTP failure with exponential backoff
- app/routers/email.py: GET /api/email/logs (paginated, filterable by type/status/school),
  POST /api/email/test (send test email, super admin)
- tasks/billing.py: invoice + overdue warning + suspension emails now use HTML templates
- tasks/sms.py: low credit alert now uses HTML template
- tasks/license.py: expiry warning now uses HTML template
- app/main.py + migrations/env.py: wire in email_log model + email router

Frontend:
- EmailLogsPage.vue: table with to/subject/type badge/status badge/sent_at/error,
  type+status filters, pagination, Send Test Email modal
- router/index.ts: /email-logs route
- AppSidebar.vue: Email Logs nav item
- api.ts: getEmailLogs, sendTestEmail

frontend/src/router/index.ts

@@ -71,6 +71,12 @@ const router = createRouter({
       component: () => import('@/pages/AnnouncementsPage.vue'),
       meta: { requiresAuth: true, layout: 'app', role: 'super_admin' },
     },
+    {
+      path: '/email-logs',
+      name: 'email-logs',
+      component: () => import('@/pages/EmailLogsPage.vue'),
+      meta: { requiresAuth: true, layout: 'app', role: 'super_admin' },
+    },
     // School Portal routes
     {
       path: '/portal',