feat: additional receipts as attachments to teamer invoices

addresses #869dv97u9

src/Entity/Disposition.php

@@ -188,6 +188,20 @@ class Disposition implements BlameableEntityInterface, TimestampableEntityInterf
         return null;
     }
 
+    /**
+     * Unlike getDocumentByType() this returns every match, for the types a disposition can hold
+     * more than one of. Belege are the only such type today: there is exactly one Honorarvertrag
+     * and one Honorarnote, but any number of receipts backing the latter.
+     *
+     * @return Collection<int, Upload>
+     */
+    public function getDocumentsByType(string $type): Collection
+    {
+        return $this->documents->filter(function (Upload $upload) use ($type) {
+            return $type === $upload->getType();
+        });
+    }
+
     public function addDocument(Upload $document): static
     {
         if (!$this->documents->contains($document)) {