feat(jobs): implement exponential backoff for unreachable servers

Reduce load on unreachable servers by implementing exponential backoff
during connectivity failures. Check frequency decreases based on
consecutive failure count:
  0-2: every cycle
  3-5: ~15 min intervals
  6-11: ~30 min intervals
  12+: ~60 min intervals

Uses server ID hash to distribute checks across cycles and prevent
thundering herd.

ServerCheckJob and ServerConnectionCheckJob increment unreachable_count
on failures. ServerManagerJob applies backoff logic before dispatching
checks. Includes comprehensive test coverage.

app/Jobs/ServerCheckJob.php

@@ -15,6 +15,7 @@ use Illuminate\Foundation\Bus\Dispatchable;
 use Illuminate\Queue\InteractsWithQueue;
 use Illuminate\Queue\Middleware\WithoutOverlapping;
 use Illuminate\Queue\SerializesModels;
+use Illuminate\Queue\TimeoutExceededException;
 use Illuminate\Support\Facades\Log;
 
 class ServerCheckJob implements ShouldBeEncrypted, ShouldQueue
@@ -36,11 +37,12 @@ class ServerCheckJob implements ShouldBeEncrypted, ShouldQueue
 
     public function failed(?\Throwable $exception): void
     {
-        if ($exception instanceof \Illuminate\Queue\TimeoutExceededException) {
+        if ($exception instanceof TimeoutExceededException) {
             Log::warning('ServerCheckJob timed out', [
                 'server_id' => $this->server->id,
                 'server_name' => $this->server->name,
             ]);
+            $this->server->increment('unreachable_count');
 
             // Delete the queue job so it doesn't appear in Horizon's failed list.
             $this->job?->delete();