feat(phase-8): billing engine + invoice PDF + mark-paid + overdue escalation

Backend:
- app/services/invoice_pdf.py: Jinja2+WeasyPrint PDF generation, saves to
  /app/data/invoices/{id}.pdf, updates Invoice.pdf_path
- app/templates/invoice.html: professional branded A4 invoice template with
  school details, line items table, totals, payment instructions, paid receipt
- routers/billing.py: GET /invoices/{id}/pdf (auto-generate on demand, FileResponse),
  POST /invoices/{id}/mark-paid (payment_method + reference → status=paid),
  POST /trigger-generate-invoices (manual trigger), school_name in invoice list
- tasks/billing.py: fix missing func import in generate_monthly_invoices,
  new billing.generate_invoice_pdf Celery task, auto-send email after
  invoice creation, check_overdue upgraded with 7-day warning emails and
  30-day school suspension + suspension email

Frontend:
- BillingPage.vue: full rewrite — status filter tabs, school names (not UUIDs),
  PDF download button, mail icon, Mark Paid modal with method/reference fields,
  overdue rows highlighted, pagination, Generate Invoices trigger button
- api.ts: markInvoicePaid, downloadInvoicePdf, triggerGenerateInvoices

.paul/ROADMAP.md

@@ -19,8 +19,8 @@ SMS flow: On-prem TapTrack polls Hub every 30s → Hub queues SMS jobs → Hub s
 ## Current Milestone
 
 **v1.0 — Foundation & Core Services**
-Status: Phase 7 complete — Phase 8 next
-Phases: 7 of 15 complete
+Status: Phase 8 complete — Phase 9 next
+Phases: 8 of 15 complete
 
 ---
 
@@ -35,7 +35,7 @@ Phases: 7 of 15 complete
 | 5     | On-Prem SMS Polling Agent      | 1   | ✅ Complete  | 2026-03-16 |
 | 6     | Super Admin Dashboard UI       | 1   | ✅ Complete  | 2026-03-16 |
 | 7     | School Admin Portal UI         | 1   | ✅ Complete  | 2026-03-16 |
-| 8     | Billing Engine + Invoice PDF   | TBD | Not started  | —          |
+| 8     | Billing Engine + Invoice PDF   | 1   | ✅ Complete  | 2026-03-16 |
 | 9     | Email Dispatcher               | TBD | Not started  | —          |
 | 10    | Support Ticket System          | TBD | Not started  | —          |
 | 11    | Monthly Report Generation      | TBD | Not started  | —          |