M5: retention policy, prune with repacking, append-only locations

- RetentionPolicy (last/daily/weekly/monthly) and PruneService: forget
  snapshots the policy drops, delete packs nothing uses and repack mostly
  unused ones by copying encrypted blobs as-is; new packs are indexed
  before old ones are removed
- a location lock keeps prune and backups apart; append-only locations
  are never pruned (cleanup via the location's lifecycle rules)
- weekly prune from the background job; occ backup:prune [--dry-run],
  backup:retention, backup:target:append-only
- tests: forget+prune frees space and the remaining snapshot restores
  identically, repack of mixed packs, dry-run preview, retention buckets

tests/Unit/Service/RetentionPolicyTest.php

@@ -0,0 +1,32 @@
+<?php
+
+declare(strict_types=1);
+
+namespace OCA\NgBackup\Tests\Unit\Service;
+
+use OCA\NgBackup\Service\RetentionPolicy;
+use PHPUnit\Framework\TestCase;
+
+class RetentionPolicyTest extends TestCase {
+	public function testDailyWeeklyMonthly(): void {
+		$snaps = [];
+		$start = strtotime('2026-01-01 03:00:00');
+		for ($d = 0; $d < 120; $d++) {
+			$snaps["d$d"] = $start + $d * 86400;          // one per day for 120 days
+			$snaps["d$d-b"] = $start + $d * 86400 + 3600; // and a second one an hour later
+		}
+		$keep = (new RetentionPolicy(2, 7, 4, 3))->keep($snaps);
+		$this->assertArrayHasKey('d119-b', $keep, 'newest is always kept');
+		$this->assertArrayHasKey('d119', $keep, 'last=2');
+		$this->assertContains('daily', $keep['d113-b']);
+		$this->assertArrayNotHasKey('d113', $keep, 'only the newest of a day');
+		$this->assertLessThanOrEqual(2 + 7 + 4 + 3, count($keep));
+		$months = array_unique(array_map(fn ($id) => date('Y-m', $snaps[$id]), array_keys(array_filter($keep, fn ($r) => in_array('monthly', $r, true)))));
+		$this->assertCount(3, $months);
+	}
+
+	public function testKeepsEverythingWhenFewSnapshots(): void {
+		$keep = (new RetentionPolicy())->keep(['a' => 100, 'b' => 200]);
+		$this->assertSame(['a', 'b'], array_values(array_intersect(['a', 'b'], array_keys($keep))));
+	}
+}