Fix remaining PR review comments across packaging files

- preremove.sh: Stop the systemd service during upgrades (not just
  remove/purge) so the running process doesn't use stale venv binaries
  while postinstall replaces them. Only disable + remove venv on full
  removal.

- debian.yml: Fail loudly when release lookup fails during a release
  event (exit 1), but still skip gracefully for workflow_dispatch
  manual testing (exit 0). Prevents silently broken .deb publication.

- archivebox.rb + build_brew.sh + homebrew.yml: Add post_install that
  initializes ArchiveBox in var/archivebox with DATA_DIR set, using
  correct Homebrew system() syntax (no env hash as first arg).

https://claude.ai/code/session_01Vx1EsNrNySgsc8Y67dGzCn

.github/workflows/homebrew.yml

@@ -94,6 +94,13 @@ ${RESOURCES}
     virtualenv_install_with_resources
   end
 
+  def post_install
+    data_dir = var/"archivebox"
+    data_dir.mkpath
+    ENV["DATA_DIR"] = data_dir.to_s
+    system bin/"archivebox", "init"
+  end
+
   test do
     assert_match version.to_s, shell_output("#{bin}/archivebox version")
   end