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/sms.py

@@ -146,7 +146,20 @@ def send_low_credit_alert(school_id: str):
             send_email(
                 to=school.billing_email,
                 subject=f"[TapTrack Hub] Low SMS Credits — {school.name}",
-                body=f"Your SMS credit balance for {school.name} is low ({float(school.sms_credits):.0f} remaining). Please top up to continue sending SMS notifications.",
+                body=(
+                    f"Your SMS credit balance for {school.name} is low "
+                    f"({float(school.sms_credits):.0f} credits remaining). "
+                    f"Please top up to continue sending SMS notifications."
+                ),
+                template_name="email/low_credit.html",
+                context={
+                    "contact_name": school.contact_name or school.name,
+                    "school_name": school.name,
+                    "credits_remaining": int(float(school.sms_credits)),
+                    "threshold": school.sms_credit_low_threshold,
+                },
+                email_type="low_credit",
+                school_id=school.id,
             )
     finally:
         db.close()