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
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