Graph the metric behind each alert on the incident page

Each alert carries a generatorURL pointing at the Prometheus rule that fired
it. The server now parses that link, queries the same Prometheus for the
rule's expression and draws it under the alert, with the current value
beside it. The browser never talks to Prometheus, so the CSP stays at
connect-src 'self'.

The URL comes out of a webhook payload, so whoever holds an integration key
chooses it. Querying it is therefore opt-in and bounded by an allowlist,
TERDUT_PROMETHEUS_ALLOWED_HOSTS (names, "*" as one label, a leading "**" as
several, IPs, CIDRs). The check runs on the address the dialer is about to
use, a name that resolves to loopback or link-local is refused even when it
matches, and redirects and proxy variables are ignored.

A worker takes a snapshot when an incident opens and another when it
closes, so the graph outlives Prometheus' retention. It finds its work in
the database rather than being triggered from the webhook, so ingestion is
untouched and a restart loses nothing. While an incident is open the
metrics endpoint also queries live, cached for 15 seconds.

The rule expression keeps its threshold, so Prometheus returns data only
while the alert condition holds; "no data" for the current value means the
condition cleared. Stripping the threshold needs a PromQL parser and is left
out.

Claude-Session: https://claude.ai/code/session_016mBLURvJoMuUEr9cB2RpUN

charts/terdut-server/templates/deployment.yaml

@@ -120,6 +120,14 @@ spec:
               value: {{ .Values.trustedProxies | quote }}
             - name: TERDUT_OPERATOR_MODE
               value: {{ .Values.operatorMode | quote }}
+            {{- if .Values.prometheus.allowedHosts }}
+            - name: TERDUT_PROMETHEUS_ALLOWED_HOSTS
+              value: {{ join "," .Values.prometheus.allowedHosts | quote }}
+            - name: TERDUT_PROMETHEUS_PREROLL
+              value: {{ .Values.prometheus.preroll | quote }}
+            - name: TERDUT_PROMETHEUS_TIMEOUT
+              value: {{ .Values.prometheus.timeout | quote }}
+            {{- end }}
             {{- if .Values.oidc.enabled }}
             - name: TERDUT_OIDC_ISSUER
               value: {{ required "oidc.issuer is required when oidc.enabled" .Values.oidc.issuer | quote }}

charts/terdut-server/values.yaml

@@ -114,6 +114,18 @@ passwordLogin: true
 # configuration from here on.
 operatorMode: false
 
+# Metrics on incidents: the server queries the Prometheus an alert came from
+# (the host in its generatorURL) and graphs the alert's expression. The URL is
+# taken from a webhook payload, so only hosts listed here are ever contacted;
+# empty turns the feature off. Entries are host names, with "*" as one DNS label
+# and a leading "**" as one or more (e.g. "*.monitoring.svc.cluster.local",
+# "prometheus-*.monitoring"), IPs or CIDR ranges, each with an optional ":port".
+prometheus:
+  allowedHosts: []
+  # How far before an alert started its graph begins.
+  preroll: 1h
+  timeout: 10s
+
 # Single sign-on through an OpenID Connect provider such as Authentik.
 #
 # At the provider, create an OAuth2/OpenID application whose redirect URI is