Implements the "1-Klick-PDF-Export für Handwerker" requirement from
the ticketing module spec, which existed only as a schema stub
(HandoverProtocol.pdfUrl) with no actual generator anywhere in the
codebase. Adds a server-side PDF endpoint (pdfkit, no headless
browser needed) summarizing a ticket's category, priority, status,
room, reporter and description, plus a "PDF" button in the landlord
cockpit that downloads it via an authenticated blob fetch (the API
uses a Bearer token, not cookies, so a plain <a href> wouldn't carry
auth).
