Move files, complete PHP migration

The application works the same way as the F# version

src/lib/Domain/Recurrence.php

@@ -0,0 +1,41 @@
+<?php declare(strict_types=1);
+
+namespace MyPrayerJournal\Domain;
+
+use DateInterval;
+use JsonSerializable;
+
+/**
+ * The recurrence for a prayer request
+ */
+class Recurrence implements JsonSerializable
+{
+    /**
+     * @param RecurrencePeriod $period The recurrence period
+     * @param int|null $interval How many of the periods will pass before the request is visible again
+     */
+    public function __construct(public RecurrencePeriod $period, public ?int $interval = null) { }
+
+    /**
+     * Get the date/time interval for this recurrence
+     *
+     * @return DateInterval The interval matching the recurrence
+     */
+    public function interval(): DateInterval
+    {
+        $period = match ($this->period) {
+            RecurrencePeriod::Immediate => 'T0S',
+            RecurrencePeriod::Hours     => "T{$this->interval}H",
+            RecurrencePeriod::Days      => "{$this->interval}D",
+            RecurrencePeriod::Weeks     => ($this->interval * 7) . 'D'
+        };
+        return new DateInterval("P$period");
+    }
+
+    public function jsonSerialize(): mixed
+    {
+        $values = ['period' => $this->period->value];
+        if (isset($this->interval)) $values['interval'] = $this->interval;
+        return $values;
+    }
+}