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

backend/app/tasks/license.py

@@ -6,6 +6,7 @@ from app.worker import celery_app
 
 logger = logging.getLogger(__name__)
 
+
 @celery_app.task(name="license.check_expiry")
 def check_expiry():
     """Send expiry warning emails for licenses expiring in 30, 14, or 7 days."""
@@ -34,7 +35,20 @@ def check_expiry():
                     send_email(
                         to=school.billing_email,
                         subject=f"[TapTrack Hub] License expires in {days_ahead} days — {school.name}",
-                        body=f"Your TapTrack license for {school.name} expires on {lic.expires_at}. Please contact us to renew.",
+                        body=(
+                            f"Your TapTrack license for {school.name} expires on {lic.expires_at} "
+                            f"({days_ahead} days remaining). Please contact us to renew."
+                        ),
+                        template_name="email/license_expiry.html",
+                        context={
+                            "contact_name": school.contact_name or school.name,
+                            "school_name": school.name,
+                            "license_key": lic.key,
+                            "expires_at": str(lic.expires_at),
+                            "days_left": days_ahead,
+                        },
+                        email_type="license_expiry",
+                        school_id=school.id,
                     )
         db.commit()
     finally: