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/main.py

@@ -5,10 +5,10 @@ from fastapi.middleware.cors import CORSMiddleware
 
 from app.database import engine, Base
 # Import all models so Alembic/SQLAlchemy picks them up
-from app.models import user, school, license, sms, billing, ticket, audit, announcement  # noqa
+from app.models import user, school, license, sms, billing, ticket, audit, announcement, email_log  # noqa
 
 from app.routers import auth, schools, licenses, sms as sms_router, billing as billing_router
-from app.routers import tickets, users, dashboard, school_portal, announcements, sync
+from app.routers import tickets, users, dashboard, school_portal, announcements, sync, email as email_router
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
@@ -44,6 +44,7 @@ app.include_router(dashboard.router)
 app.include_router(school_portal.router)
 app.include_router(announcements.router)
 app.include_router(sync.router)
+app.include_router(email_router.router)
 
 @app.get("/api/health")
 async def health():