feat: support multiple services per appointment (checkbox selection)

Appointments could only reference a single service (ManyToOne). Add an
appointment_service_items join table (ManyToMany) so an appointment can
carry several services; the first stays the primary service_item for
backward compatibility, and toArray now also returns service_items[].

Both create endpoints (my/appointment, admin/appointment) accept
service_item_uuids[] and attach all of them. A new duration_from_services
flag gates the slot_end recompute: service-booking mode sends it true
(slot_end = start + Σ durations); slot mode omits it so the manual end
time is preserved. The admin endpoint previously ignored services entirely.

Frontend: in slot mode the single service dropdown becomes a checkbox list
filtered by the selected section (multi-select); service mode sends the
duration flag. Migration + backend/entity tests + docs updated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

migrations/Version20260716063302.php

@@ -0,0 +1,35 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+/**
+ * Auto-generated Migration: Please modify to your needs!
+ */
+final class Version20260716063302 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Add appointment_service_items join table for multi-service appointments';
+    }
+
+    public function up(Schema $schema): void
+    {
+        // this up() migration is auto-generated, please modify it to your needs
+        $this->addSql('CREATE TABLE appointment_service_items (appointment_id INT NOT NULL, service_item_id INT NOT NULL, INDEX IDX_3624BA00E5B533F9 (appointment_id), INDEX IDX_3624BA00DDEB00C2 (service_item_id), PRIMARY KEY (appointment_id, service_item_id)) DEFAULT CHARACTER SET utf8mb4');
+        $this->addSql('ALTER TABLE appointment_service_items ADD CONSTRAINT FK_3624BA00E5B533F9 FOREIGN KEY (appointment_id) REFERENCES appointments (id) ON DELETE CASCADE');
+        $this->addSql('ALTER TABLE appointment_service_items ADD CONSTRAINT FK_3624BA00DDEB00C2 FOREIGN KEY (service_item_id) REFERENCES service_items (id) ON DELETE CASCADE');
+    }
+
+    public function down(Schema $schema): void
+    {
+        // this down() migration is auto-generated, please modify it to your needs
+        $this->addSql('ALTER TABLE appointment_service_items DROP FOREIGN KEY FK_3624BA00E5B533F9');
+        $this->addSql('ALTER TABLE appointment_service_items DROP FOREIGN KEY FK_3624BA00DDEB00C2');
+        $this->addSql('DROP TABLE appointment_service_items');
+    }
+}