feat(sentinel): make sentinel mandatory on regular servers

Remove the enable/disable toggle from the server UI, logs page, and
Sentinel API so is_sentinel_enabled is derived and read-only. Enable
existing regular servers via migration, start Sentinel after validate-
and-install, and drop the daily ServerManagerJob restart.

database/migrations/2026_09_08_202212_enable_sentinel_for_existing_regular_servers.php

@@ -0,0 +1,31 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Support\Facades\DB;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        DB::table('server_settings')
+            ->where('is_sentinel_enabled', false)
+            ->where('is_build_server', false)
+            ->where('is_swarm_manager', false)
+            ->where('is_swarm_worker', false)
+            ->where('force_disabled', false)
+            ->where('is_reachable', true)
+            ->where('is_usable', true)
+            ->update(['is_sentinel_enabled' => true]);
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        // Existing values cannot be distinguished from values enabled before this migration.
+    }
+};