feat(server): say at startup when alerting is unconfigured (#107)

buildNotifier returns a nil interface when BRAIN_NTFY_URL is unset, and
every consumer skips a nil notifier silently. That is right for an
unconfigured homelab and wrong for an alert whose entire value is that
someone finds out: "configured and working" and "never configured" were
byte-identical in behaviour — no error, no warning, only a message that
never arrives, which nobody notices because nobody is waiting for an alert
they have never seen.

Two alerts were dark in production for exactly that reason (#100): the
governance-rewrite alert from #97, and the audit-degradation alert that
reports the audit trail is down. Both were found by firing a real event
and watching nothing happen, not by reading anything.

WARN rather than fatal, deliberately. The brain rule
`empty-credential-must-refuse-to-start-not-switch-modes` says an empty
credential must be a startup error rather than a silent mode switch — but
that is about a control being removed. Here observability degrades, no gate
is bypassed, and refusing to boot would break the unconfigured-homelab case
the nil notifier exists to support. What that rule demands and this
provides is that the mode switch stops being silent.

buildAuditSink already announces its own fallback ("capture audit: slog
sink (BRAIN_LOKI_URL unset)"), so only the notifier needed this.

alertingWarnings is pure and env-injected: the reason this gap survived is
that nothing about it was observable, so the fix is tested rather than
asserted.

Refs #107, #100, #97.

ingestion/cmd/server/main.go

@@ -147,6 +147,37 @@ func envInt(key string, fallback int) int {
 // buildNotifier returns the ntfy notifier, or a nil interface when
 // BRAIN_NTFY_URL is unset. A nil *interface* (not a typed-nil) is required so
 // DegradingSink/Reconcile and the governance observer skip it cleanly.
+// alertingWarnings names every alert that will not fire under the current
+// configuration (#107).
+//
+// buildNotifier returns a nil interface when BRAIN_NTFY_URL is unset, and every
+// consumer skips a nil notifier silently — right for an unconfigured homelab,
+// wrong for an alert whose entire value is that someone finds out. The cost of
+// the silence is that "configured and working" and "never configured" are
+// byte-identical in behaviour: no error, no warning, only a message that never
+// arrives, which nobody notices because nobody is waiting for an alert they
+// have never seen. Two alerts were dark in production for exactly that reason
+// (#100), found by firing a real event rather than by reading anything.
+//
+// WARN rather than fatal, deliberately. The brain sibling rule
+// (`empty-credential-must-refuse-to-start-not-switch-modes`) says an empty
+// credential must be a startup error, never a silent mode switch — but that is
+// about a control being removed. Here observability is degraded, not a gate
+// bypassed, and refusing to boot would break the unconfigured-homelab case the
+// nil notifier exists to support. What the rule does demand, and what this
+// provides, is that the mode switch stops being silent.
+//
+// Pure and env-injected so the message is testable without a running server:
+// the reason this gap survived is that nothing about it was observable.
+func alertingWarnings(getenv func(string) string) []string {
+	var out []string
+	if getenv("BRAIN_NTFY_URL") == "" {
+		out = append(out, "alerting DISABLED: BRAIN_NTFY_URL unset — "+
+			"brain_edit_root governance rewrites and capture audit degradation will not notify")
+	}
+	return out
+}
+
 func buildNotifier() audit.Notifier {
 	if n := audit.NewNtfyNotifier(os.Getenv("BRAIN_NTFY_URL"), os.Getenv("BRAIN_NTFY_TOKEN")); n != nil {
 		return n
@@ -203,6 +234,10 @@ func systemHostname() string {
 func main() {
 	logger := slog.New(slog.NewJSONHandler(os.Stdout, nil))
 
+	for _, w := range alertingWarnings(os.Getenv) {
+		logger.Warn(w)
+	}
+
 	brainDir := envOr("INGEST_BRAIN_DIR", "../brain")
 	port := envOr("INGEST_PORT", "3300")