chore(pkg): lintian sweep — fix obsolete polkit dep, drop dup systemd path, command -v in pam hooks; override deferred manpages and PPA-only warnings

Real fixes:
- authforge-daemon Depends: policykit-1 -> polkitd | policykit-1
  (policykit-1 was renamed; old name is obsolete on noble).
- debian/rules: drop manual install of authforge-daemon.service to
  /lib/systemd/system/ — dh_installsystemd auto-installs to
  /usr/lib/systemd/system/, the manual line caused a file-in-root-and-usr
  duplicate flagged by usrmerge.
- authforge-pam.{postinst,prerm}: replace [ -x /usr/sbin/pam-auth-update ]
  with command -v pam-auth-update — lintian flags hard-coded paths in
  test syntax; command -v is the portable idiom.

Overrides (with rationale in each file):
- no-manual-page on the three binaries — manpages deferred to Phase 18.
- desktop-command-not-in-package authforge in gnome-integration —
  command lives in authforge-gui (cross-package Depends).
- initial-upload-closes-no-bugs on every package — AuthForge ships
  via PPA, not the Debian archive, so there is no ITP to close.

After fixes, lintian on the five binary debs is silent (zero W/E).

debian/authforge-gnome-integration.lintian-overrides

@@ -0,0 +1,7 @@
+# The Users-panel desktop entry launches `authforge`, which is shipped
+# by the authforge-gui binary package (Depends: authforge-gui). Lintian
+# only inspects files inside this package and can't see the cross-
+# package binary, so the warning is a false positive.
+authforge-gnome-integration: desktop-command-not-in-package authforge [usr/share/applications/io.dangerousthings.AuthForge.UsersPanel.desktop]
+# Not destined for the Debian archive — see authforge.lintian-overrides.
+authforge-gnome-integration: initial-upload-closes-no-bugs