feat(grafana): add HAProxy requests dashboard and Richie Postgres datasource

Visualize the ingested HAProxy request logs in Grafana.

- add a `richie-postgres` datasource connecting to the Richie DB over the
  local Unix socket (/run/postgresql; pg_hba trust for user richie)
- order grafana after postgresql.service
- add the "HAProxy Requests" dashboard: request rate by backend, response
  -time percentiles (p50-p99), top user-agents/IPs/endpoints, status mix

systems/jeeves/services/grafana.nix

@@ -62,6 +62,21 @@ in
             uid = "prom-pid-short";
             url = "http://127.0.0.1:9092";
           }
+          {
+            access = "proxy";
+            editable = false;
+            name = "richie-postgres";
+            type = "postgres";
+            uid = "richie-postgres";
+            url = "/run/postgresql";
+            user = "richie";
+            jsonData = {
+              database = "richie";
+              sslmode = "disable";
+              postgresVersion = 1700;
+              timescaledb = false;
+            };
+          }
         ];
       };
     };
@@ -71,6 +86,7 @@ in
     services.grafana.after = [
       "prometheus-main.service"
       "prometheus-pid-short.service"
+      "postgresql.service"
     ];
 
     tmpfiles.rules = [