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

backend/app/services/invoice_pdf.py

@@ -0,0 +1,70 @@
+"""Invoice PDF generation using Jinja2 + WeasyPrint."""
+import logging
+import os
+from datetime import datetime, timezone
+from pathlib import Path
+
+logger = logging.getLogger(__name__)
+
+# PDF output directory (mounted as a Docker volume in production)
+PDF_DIR = Path(os.getenv("PDF_DIR", "/app/data/invoices"))
+
+
+def generate_invoice_pdf(invoice_id: str, db_session) -> str:
+    """
+    Render the invoice HTML template and convert to PDF via WeasyPrint.
+
+    Args:
+        invoice_id: UUID of the Invoice to generate
+        db_session: synchronous SQLAlchemy session (used from Celery tasks)
+
+    Returns:
+        Absolute path to the generated PDF file.
+    """
+    from app.models.billing import Invoice, InvoiceLineItem
+    from app.models.school import School
+    from jinja2 import Environment, FileSystemLoader
+    from weasyprint import HTML
+
+    # Load invoice + line items + school
+    inv = db_session.get(Invoice, invoice_id)
+    if not inv:
+        raise ValueError(f"Invoice {invoice_id} not found")
+
+    school = db_session.get(School, inv.school_id)
+    line_items = db_session.query(InvoiceLineItem).filter_by(invoice_id=invoice_id).all()
+
+    # Format dates
+    def _fmt(d) -> str:
+        if not d:
+            return ""
+        if hasattr(d, "strftime"):
+            return d.strftime("%B %d, %Y")
+        return str(d)
+
+    context = {
+        "invoice": inv,
+        "school": school,
+        "line_items": line_items,
+        "issued_date": _fmt(inv.created_at),
+        "period_start": _fmt(inv.billing_period_start),
+        "period_end": _fmt(inv.billing_period_end),
+        "due_date": _fmt(inv.due_date),
+        "paid_date": _fmt(inv.paid_at),
+        "generated_date": datetime.now(timezone.utc).strftime("%Y-%m-%d %H:%M UTC"),
+    }
+
+    # Render Jinja2 template
+    template_dir = Path(__file__).parent.parent / "templates"
+    env = Environment(loader=FileSystemLoader(str(template_dir)), autoescape=True)
+    template = env.get_template("invoice.html")
+    html_content = template.render(**context)
+
+    # Generate PDF
+    PDF_DIR.mkdir(parents=True, exist_ok=True)
+    output_path = PDF_DIR / f"{invoice_id}.pdf"
+
+    HTML(string=html_content).write_pdf(str(output_path))
+    logger.info("Generated PDF: %s", output_path)
+
+    return str(output_path)