feat: answer received form submissions by mail

src/Model/ApplicationFormSubmissionDto.php

@@ -66,6 +66,12 @@ class ApplicationFormSubmissionDto
         public readonly ?string $zipCode = null,
         #[Assert\Length(max: 255)]
         public readonly ?string $city = null,
+        // Whether the applicant holds an EU (incl. Switzerland) nationality - the one payload
+        // field a rule depends on, hence mapped rather than left in $details. bool|string|int
+        // for the same reason as pageUid: a form posts "1" or "" as readily as a JSON boolean,
+        // and losing a whole application over the spelling would be absurd. Read it back
+        // through isEu().
+        public readonly bool|string|int|null $eu = null,
         public readonly array $details = [],
     ) {
     }
@@ -113,6 +119,28 @@ class ApplicationFormSubmissionDto
      * Only the shape is checked, because that is all the json column and the display can carry:
      * a label with content, and a scalar or a flat list as the answer.
      */
+    /**
+     * A value that is neither a boolean nor one of the spellings a form posts is a 400 rather
+     * than a silent null: null means "this form does not ask", and a misconfigured mapping that
+     * quietly stopped every receipt mail would be the worst of both worlds. The same reasoning
+     * as validatePageUid().
+     */
+    #[Assert\Callback]
+    public function validateEu(ExecutionContextInterface $executionContext): void
+    {
+        if (null === $this->eu) {
+            return;
+        }
+
+        if (null === $this->isEu()) {
+            $executionContext
+                ->buildViolation('eu must be a boolean.')
+                ->atPath('eu')
+                ->addViolation()
+            ;
+        }
+    }
+
     #[Assert\Callback]
     public function validateDetails(ExecutionContextInterface $executionContext): void
     {
@@ -154,6 +182,25 @@ class ApplicationFormSubmissionDto
         }
     }
 
+    /**
+     * null for "the form does not ask" as well as for a value that made no sense - the caller
+     * treats both as "no automatic mail", and validateEu() has already turned the second into a
+     * 400 by the time anything reads this.
+     */
+    public function isEu(): ?bool
+    {
+        if (is_bool($this->eu)) {
+            return $this->eu;
+        }
+
+        return match (is_string($this->eu) ? mb_strtolower(trim($this->eu)) : $this->eu) {
+            null => null,
+            1, '1', 'true' => true,
+            0, '0', 'false', '' => false,
+            default => null,
+        };
+    }
+
     public function getPageUid(): ?int
     {
         if (null === $this->pageUid) {