swarm UI: an alerts page, read by the controller with a Viewer Grafana token from bao

swarm-bao-grafana-controller-token, on the host that runs Grafana, logs in
to Grafana's API over its socket as the built-in admin (the password
swarm-bao-grafana-admin minted, #5138) and keeps a `swarm-controller`
service account that is an enabled Viewer. It mints a token for that
account and writes it to bao at
swarm/controller/swarm-controller/grafana/token under Grafana's leaf. A
ten-minute timer runs it again. The unit keeps a stored token Grafana
accepts. If Grafana refuses the stored token with 401, or bao holds
none, the unit deletes every token of the account and mints a new one.
Grafana's grant gains create/read/update on that leaf, and the
controller's gains read.

swarm-controller serves GET /api/alerts from Grafana's
/api/prometheus/grafana/api/v1/alerts. It reads the token from bao,
keeps it in memory, and re-reads bao once on a 401. The HTTP client has
5s connect and 15s request timeouts. Any failure is a 503 that names its
cause. swarm-ui gets an /alerts page listing them, built the same way as
the hives page.

This is option (c) on #5129, the operator's choice.

swarm-controller/src/github_account.rs

@@ -138,6 +138,7 @@ mod tests {
             try_outs: super::super::review_classifier::TryOuts::default(),
             reviews: std::sync::Arc::default(),
             runs: crate::review_classifier::runs::Runs::Live(None),
+            alerts: None,
         }
     }