chore(cluster): cleanup trailing spaces and template consistency

[philippemnoel]

I caught these while working on the @paradedb fork. Figured these changes would benefit upstream and reduce our diff:

• Trailing whitespace after SELECT in the custom-queries example
• Extra whitespace and inconsistent context variable ($ vs $context) in scheduled-backups.yaml, plus labels not properly using nindent
• Double spaces after -- in two values.yaml comments

Enjoy!