FATAL: password authentication failed for user "postgres"

I noticed this while working on !854 (merged). This already happens on stackspin.net (pg10):

 2022-02-08 15:10:07.442 UTC [1] LOG:  listening on Unix socket "/var/run/postgresql/.s.PGSQL.5432"                                                               
 2022-02-08 15:10:07.475 UTC [21] LOG:  database system was shut down at 2022-02-08 15:09:57 UTC                                                                  
 2022-02-08 15:10:07.499 UTC [1] LOG:  database system is ready to accept connections                                                                             
 2022-02-08 15:10:44.456 UTC [51] FATAL:  password authentication failed for user "postgres"                                                                      
 2022-02-08 15:10:44.456 UTC [51] DETAIL:  Role "postgres" does not exist.                                                                                        
     Connection matched pg_hba.conf line 95: "host all all all md5"                                                                                               
 2022-02-10 15:13:27.764 UTC [271132] FATAL:  password authentication failed for user "postgres"                                                                  
 2022-02-10 15:13:27.764 UTC [271132] DETAIL:  Role "postgres" does not exist.                                                                                    
     Connection matched pg_hba.conf line 95: "host all all all md5"                                                                                               
 2022-02-17 13:42:03.093 UTC [1201520] FATAL:  password authentication failed for user "postgres"                                                                 
 2022-02-17 13:42:03.093 UTC [1201520] DETAIL:  Role "postgres" does not exist.                                                                                   
     Connection matched pg_hba.conf line 95: "host all all all md5"                                                                                               
 2022-02-18 02:57:54.341 UTC [25] LOG:  using stale statistics instead of current ones because stats collector is not responding                                  
                                                                                                                                                                  

I wonder how zulip persists chat data when it can't connect to postgres ?