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/Notifications/NudgeNotification.php

@@ -0,0 +1,53 @@
+<?php
+
+namespace App\Notifications;
+
+use App\Enums\NotificationType;
+use App\Mail\NudgeNotificationMail;
+use App\Models\Declaration;
+use App\Models\User;
+use Illuminate\Bus\Queueable;
+use Illuminate\Contracts\Queue\ShouldQueue;
+use Illuminate\Notifications\Notification;
+
+class NudgeNotification extends Notification implements ShouldQueue
+{
+    use Queueable;
+
+    public int $tries = 3;
+
+    public int $backoff = 60;
+
+    public bool $deleteWhenMissingModels = true;
+
+    public function __construct(
+        public Declaration $declaration,
+        public User $sender,
+    ) {}
+
+    /**
+     * @return array<string>
+     */
+    public function via(object $notifiable): array
+    {
+        return ['database', 'mail'];
+    }
+
+    /**
+     * @return array<string, mixed>
+     */
+    public function toDatabase(object $notifiable): array
+    {
+        return [
+            'workspace_id' => $this->declaration->workspace_id,
+            'declaration_id' => $this->declaration->id,
+            'sender_id' => $this->sender->id,
+            'notification_type' => NotificationType::Nudge,
+        ];
+    }
+
+    public function toMail(object $notifiable): NudgeNotificationMail
+    {
+        return new NudgeNotificationMail($this->declaration, $this->sender);
+    }
+}