fix(backup): use shared compression for database dumps

Centralize compressor selection and apply helper-image compression to dump-all database backups.

tests/Unit/BackupCompressionTest.php

@@ -0,0 +1,25 @@
+<?php
+
+use App\Support\BackupCompression;
+
+it('normalizes supported backup compression CPU percentages', function (mixed $configuredPercentage, int $expected) {
+    expect(BackupCompression::cpuPercentage($configuredPercentage))->toBe($expected);
+})->with([
+    'null defaults to low' => [null, 25],
+    'unsupported value defaults to low' => [30, 25],
+    'numeric string is accepted' => ['50', 50],
+    'low' => [25, 25],
+    'medium' => [50, 50],
+    'high' => [75, 75],
+    'maximum' => [100, 100],
+]);
+
+it('builds the shared pigz command with gzip fallback', function () {
+    $command = BackupCompression::compressorCommand(75);
+
+    expect($command)
+        ->toContain('command -v pigz')
+        ->toContain('pigz -3 -p')
+        ->toContain('$(nproc) * 75 + 99')
+        ->toContain('gzip -3');
+});