Pin a restore point and keep a note on it

Both live in the catalog on the location, so a new server sees them; the database holds
a local copy for the lists. The retention policy leaves a pinned restore point alone and
does not count it. Available on the restore page and as occ backup:pin and backup:note.

lib/Service/RetentionPolicy.php

@@ -28,6 +28,19 @@ final class RetentionPolicy {
 		return ['last' => $this->last, 'daily' => $this->daily, 'weekly' => $this->weekly, 'monthly' => $this->monthly];
 	}
 
+	/**
+	 * keep() for the snapshots that are not pinned, plus the pinned ones. A pinned snapshot does not
+	 * take the place of a daily, weekly or monthly one.
+	 *
+	 * @param array<string, int> $snapshots id => unix time
+	 * @param list<string> $pinned
+	 * @return array<string, list<string>>
+	 */
+	public function keepWithPinned(array $snapshots, array $pinned): array {
+		$pins = array_intersect_key($snapshots, array_flip($pinned));
+		return $this->keep(array_diff_key($snapshots, $pins)) + array_fill_keys(array_keys($pins), ['pinned']);
+	}
+
 	/**
 	 * @param array<string, int> $snapshots id => unix time
 	 * @return array<string, list<string>> id => reasons it is kept (ids not in the result are removed)