feat: add notification infrastructure with database channel, enum, and notification classes (Story 3.1)

Set up Laravel notification system with NotificationType enum (5 types),
NudgeNotification, DocumentUploadedNotification, and DeclarationOverdueNotification
classes with database + mail channels. Add email templates, infrastructure tests,
and fix existing NotificationController tests for workspace compatibility.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/Enums/NotificationType.php

@@ -0,0 +1,34 @@
+<?php
+
+namespace App\Enums;
+
+use BenSampo\Enum\Enum;
+
+final class NotificationType extends Enum
+{
+    const Nudge = 'nudge';
+
+    const DeclarationOverdue = 'declaration_overdue';
+
+    const DocumentUploaded = 'document_uploaded';
+
+    const BulkNotification = 'bulk_notification';
+
+    const StatusChanged = 'status_changed';
+
+    /**
+     * Get French display labels for each notification type.
+     *
+     * @return array<string, string>
+     */
+    public static function labels(): array
+    {
+        return [
+            self::Nudge => 'Relance',
+            self::DeclarationOverdue => 'Déclaration en retard',
+            self::DocumentUploaded => 'Document téléversé',
+            self::BulkNotification => 'Notification groupée',
+            self::StatusChanged => 'Statut modifié',
+        ];
+    }
+}