feat(cntr): add QR code check-in and PDF invoice generator services

[phertyameen]

Closes #952
Closes #953
Closes #954

Adds two new services under backend/cntr/:

• qrcode.service.ts - generateCheckInQRCode(workspaceId, sessionToken) generates a Base64 PNG data URL encoding a 15-minute expiring JSON payload. Throws on empty inputs.
• invoice-pdf.service.ts - generateInvoicePDF(invoice) produces a valid PDF buffer with ManageHub branding, line-items table, subtotal, 7.5% VAT, and grand total. Returns Promise<Buffer> (PDFKit is stream-based; synchronous return is not possible).

Both services use already-installed packages (qrcode, pdfkit) and include unit tests with mocking where applicable.

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