Alert when an integration or DNS provider fails repeatedly

The Diagnostic Log already records every outbound call's success or
failure, but nothing acted on it — you'd only notice an integration
was down by happening to open its page. Adds a per-source consecutive-
failure counter (in-memory, reset on restart, same durability tier as
the diag log's own ring buffer) hooked into recordDiagEntry: crossing
the configurable threshold (default 3) sends one "down" notification
on every configured channel, and a "recovered" notification fires once
it succeeds again — no repeat spam while it stays down. New
"Integration/DNS provider failing repeatedly" toggle and threshold
field under Settings -> Notifications.

Verified end-to-end against an isolated scratch database with a real
local HTTP server standing in for the webhook channel: 5 consecutive
failures produced exactly one "Down" notification (at the 3rd
failure, correctly naming "3 calls"), a subsequent success produced
exactly one "Recovered" notification, and two more failures on a
fresh streak triggered nothing (below threshold) — confirmed the real
dev database's mtime was untouched throughout.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

server/src/services/notify.ts

@@ -191,3 +191,36 @@ export async function notifyTailscaleKeyExpiry(
     `${expiring.length} device key${expiring.length !== 1 ? "s" : ""} need attention:\n\n${lines.join("\n")}`,
   );
 }
+
+// Diagnostic-log "source" values (see integrations/*/adapter.ts and dns/adapters/*.ts) to display names.
+const SOURCE_LABELS: Record<string, string> = {
+  cloudflare: "Cloudflare",
+  loopia: "Loopia",
+  pihole: "Pi-hole",
+  azure: "Azure DNS",
+  cpanel: "cPanel",
+  technitium: "Technitium",
+  tailscale: "Tailscale",
+  proxmox: "Proxmox",
+  synology: "Synology",
+  semaphore: "Semaphore",
+  gitea: "Gitea",
+  dockhand: "Dockhand",
+};
+
+function sourceLabel(source: string): string {
+  return SOURCE_LABELS[source] ?? source;
+}
+
+export async function notifyIntegrationDown(source: string, consecutiveFailures: number): Promise<void> {
+  if (!(await eventEnabled("integrationFailureAlerts"))) return;
+  await notify(
+    "Homelab Manager — Integration Down",
+    `${sourceLabel(source)} has failed its last ${consecutiveFailures} call${consecutiveFailures !== 1 ? "s" : ""} in a row. Check the Diagnostic Log for details.`,
+  );
+}
+
+export async function notifyIntegrationRecovered(source: string): Promise<void> {
+  if (!(await eventEnabled("integrationFailureAlerts"))) return;
+  await notify("Homelab Manager — Integration Recovered", `${sourceLabel(source)} succeeded again after failing.`);
+}