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/templates/email/license_expiry.html

@@ -0,0 +1,23 @@
+{% extends "email/base.html" %}
+{% block content %}
+<h2>License Expiring in {{ days_left }} Days</h2>
+<p>Dear {{ contact_name }},</p>
+
+<div class="alert-amber">
+  <strong>Your TapTrack license for {{ school_name }} expires on {{ expires_at }}.</strong>
+</div>
+
+<div class="info-box">
+  <div class="row"><span class="label">School</span><span class="value">{{ school_name }}</span></div>
+  <div class="row"><span class="label">License Key</span><span class="value" style="font-family:monospace;font-size:12px">{{ license_key }}</span></div>
+  <div class="row"><span class="label">Expires On</span><span class="value">{{ expires_at }}</span></div>
+  <div class="row"><span class="label">Days Remaining</span><span class="value" style="color:#b45309">{{ days_left }} days</span></div>
+</div>
+
+<p>When your license expires, SMS notifications will be disabled and your on-prem TapTrack
+instance will enter read-only mode. Please contact us to renew your license before it expires.</p>
+
+<p><a class="btn" href="mailto:support@taptrack.io?subject=License Renewal — {{ school_name }}">Contact Us to Renew</a></p>
+
+<p>Thank you,<br><strong>TapTrack Hub Team</strong></p>
+{% endblock %}