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  (policykit-1 was renamed; old name is obsolete on noble).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/rules

@@ -43,9 +43,8 @@ override_dh_auto_install:
 	# polkit actions
 	install -D -m 0644 debian/io.dangerousthings.AuthForge.policy \
 	    debian/authforge-daemon/usr/share/polkit-1/actions/io.dangerousthings.AuthForge.policy
-	# systemd unit (dh_installsystemd will enable it)
-	install -D -m 0644 debian/authforge-daemon.service \
-	    debian/authforge-daemon/lib/systemd/system/authforge-daemon.service
+	# systemd unit: dh_installsystemd auto-installs debian/authforge-daemon.service
+	# to /usr/lib/systemd/system/ — no manual install line needed.
 	# GNOME Settings Users-panel overlay (authforge-gnome-integration)
 	install -D -m 0644 gnome-integration/io.dangerousthings.AuthForge.UsersPanel.desktop \
 	    debian/authforge-gnome-integration/usr/share/applications/io.dangerousthings.AuthForge.UsersPanel.desktop