feat(notifications): add a consecutive-failure threshold for outbound.down alerts (#5968)

Problem: a flaky outbound produces hundreds of false-positive "outbound down"
notifications overnight — each fires the moment xray's observatory reports a
single failed probe, and the next successful probe fires an "up".

applyObservatory forwarded every raw alive:true->false transition straight to
EventOutboundDown; xray's observatory has effectively no hysteresis, and nothing
on the panel side debounced it (the email/Telegram subscribers are pure
formatters).

Fix: debounce per outbound. outbound.down now fires only after
outboundDownThreshold consecutive FAILED probes (new setting, default 3);
outbound.up fires immediately on the first successful probe and only when a down
was actually notified. The threshold gates the event itself, so email and
Telegram share one knob (exposed next to the outbound.down toggle).

The streak counts genuinely new probes (last_try_time advancing), not sampler
polls — the sampler runs every 2s but the observatory re-probes per its
probeInterval, so counting samples would trip the threshold instantly.
outboundDownThreshold=1 reproduces the legacy notify-on-first-failure behaviour.

Tuning the observatory's probe interval/timeout is not a workaround: those
probes also drive the load balancer's outbound selection, so loosening them to
quiet notifications would slow real failover away from a genuinely dead
outbound. Notifications don't need observatory-grade latency, so the tolerance
belongs at the notification layer, leaving the observatory (and balancer)
untouched.

Adds TestApplyObservatoryDebounce covering the threshold, probe-vs-sample
counting, single-blip suppression and the legacy path.

Co-authored-by: Yuriy Khachaturian <y.khachaturian@souzmult.ru>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/models/setting.ts

@@ -103,6 +103,7 @@ export class AllSetting {
   smtpEnabledEvents = '';
   smtpCpu = 80;
   smtpMemory = 80;
+  outboundDownThreshold = 3;
   hasTgBotToken = false;
   hasTwoFactorToken = false;
   hasLdapPassword = false;
@@ -120,6 +121,8 @@ export class AllSetting {
     }
     const cpu = Math.round(Number(this.tgCpu));
     this.tgCpu = Number.isFinite(cpu) ? Math.min(100, Math.max(0, cpu)) : 80;
+    const threshold = Math.round(Number(this.outboundDownThreshold));
+    this.outboundDownThreshold = Number.isFinite(threshold) ? Math.min(100, Math.max(1, threshold)) : 3;
   }
 
   equals(other: AllSetting): boolean {