merge staging into production

• Run nix-collect-garbage weekly on small-disk AWS instances (!149 (merged))
• Reduce the amount of swap on small-disk AWS instances (!150 (merged))
• Some Prometheus cleanups (!145 (merged))
• Update PaymentServer to 0.1.1.1 for metric fixes (!147 (merged))
• Add a timer service to periodically trigger the cert renewal service (!143 (merged))
• Add ssh.nix to monitoring systems (!142 (merged))