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>

web/src/pages/settings/NotificationSettings.tsx

@@ -44,6 +44,8 @@ const DEFAULT_NOTIFICATIONS: NotificationEvents = {
   tailscaleKeyCheck: true,
   secretCheckTime: "08:00",
   timezone: "UTC",
+  integrationFailureAlerts: true,
+  integrationFailureThreshold: 3,
 };
 
 type TestResult = { ok: boolean; message: string } | null;
@@ -489,6 +491,7 @@ export default function NotificationSettings() {
                 { key: "dnsDelete" as const, label: "DNS record deleted" },
                 { key: "secretCheck" as const, label: "Secret expiry reminder" },
                 { key: "tailscaleKeyCheck" as const, label: "Tailscale key expiry reminder" },
+                { key: "integrationFailureAlerts" as const, label: "Integration/DNS provider failing repeatedly" },
               ].map(({ key, label }) => (
                 <label key={key} className="form-check mb-2">
                   <input
@@ -500,6 +503,26 @@ export default function NotificationSettings() {
                   <span className="form-check-label">{label}</span>
                 </label>
               ))}
+              <div className="row g-2 mt-2">
+                <div className="col-6">
+                  <label className="form-label">Alert after</label>
+                  <div className="input-group">
+                    <input
+                      type="number"
+                      className="form-control"
+                      min={1}
+                      max={20}
+                      value={notifications.integrationFailureThreshold}
+                      disabled={!notifications.integrationFailureAlerts}
+                      onChange={(e) => setNotifications((n) => ({ ...n, integrationFailureThreshold: Number(e.target.value) }))}
+                    />
+                    <span className="input-group-text">consecutive failures</span>
+                  </div>
+                  <div className="form-hint">
+                    Based on the Diagnostic Log. A recovery notice is sent once it succeeds again.
+                  </div>
+                </div>
+              </div>
               {(() => {
                 const dailyChecksEnabled = notifications.secretCheck || notifications.tailscaleKeyCheck;
                 return (