nixos/postgresql: set Restart=always for postgresql.service

...including a slightly more careful config around restarts, i.e.

* We have intervals of 5 seconds between restarts instead of 100ms.

* If we exceed 5 start attempts in 5*120s (with 120s being the timeout),
  start job gets rate-limited and thus aborted. Do note that there are
  at most 5 start attempts allowed in ~625s by default. If the startup
  fails very quickly, either wait until the rate-limit is over or reset
  the counter using `systemctl reset-failed postgresql.service`.

* The interval of 625s (plus 5s of buffer) are automatically derived
  from RestartSec & TimeoutSec. Changing either will also affect
  StartLimitIntervalSec unless overridden with `mkForce`.

nixos/modules/services/databases/postgresql.nix

@@ -826,6 +826,8 @@ in
 
           ExecStart = "${cfg.finalPackage}/bin/postgres";
 
+          Restart = "always";
+
           # Hardening
           CapabilityBoundingSet = [ "" ];
           DevicePolicy = "closed";
@@ -877,7 +879,20 @@ in
         })
       ];
 
-      unitConfig.RequiresMountsFor = "${cfg.dataDir}";
+      unitConfig =
+        let
+          inherit (config.systemd.services.postgresql.serviceConfig) TimeoutSec;
+          maxTries = 5;
+          bufferSec = 5;
+        in
+        {
+          RequiresMountsFor = "${cfg.dataDir}";
+
+          # The max. time needed to perform `maxTries` start attempts of systemd
+          # plus a bit of buffer time (bufferSec) on top.
+          StartLimitIntervalSec = TimeoutSec * maxTries + bufferSec;
+          StartLimitBurst = maxTries;
+        };
     };
 
     systemd.services.postgresql-setup = {