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/Mail/NudgeNotificationMail.php

@@ -0,0 +1,43 @@
+<?php
+
+namespace App\Mail;
+
+use App\Models\Declaration;
+use App\Models\User;
+use Illuminate\Bus\Queueable;
+use Illuminate\Mail\Mailable;
+use Illuminate\Mail\Mailables\Content;
+use Illuminate\Mail\Mailables\Envelope;
+use Illuminate\Queue\SerializesModels;
+
+class NudgeNotificationMail extends Mailable
+{
+    use Queueable, SerializesModels;
+
+    public function __construct(
+        public Declaration $declaration,
+        public User $sender,
+    ) {}
+
+    public function envelope(): Envelope
+    {
+        return new Envelope(
+            subject: 'Relance - '.($this->declaration->title ?? 'Sans titre'),
+        );
+    }
+
+    public function content(): Content
+    {
+        return new Content(
+            markdown: 'emails.nudge-notification',
+            with: [
+                'senderName' => $this->sender->name,
+                'clientName' => $this->declaration->client?->name,
+                'declarationType' => $this->declaration->type?->value,
+                'dueDate' => $this->declaration->due_date?->format('d/m/Y'),
+                'url' => route('declarations.show', $this->declaration),
+                'firmName' => $this->declaration->workspace?->name,
+            ]
+        );
+    }
+}