Two fixes for the printer install-batch on prod data:
1. Vendor was read only from the printer's direct vendorid, which the legacy
import never sets (it sets the model; legacy resolved vendor through the
model). Every prod printer came back vendor "unknown", so all fell into the
manual group and the universal PrinterInstaller.exe block never emitted. Now
resolve vendor via the model's vendor when the printer has no direct one, as
the classic installprinter.asp did.
2. Harden the download base URL. Behind IIS the app sees http on a loopback
port and url_root drops the /shopdb mount, giving a broken download URL when
site_base_url is unset. Fall back to https + the forwarded Host + script_root.
Test: a printer with no vendorid but an HP/Xerox model now groups universal.
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