feat: Add Discord alert webhook, ContactCategory enum, form request

- Added Discord alert webhook URL to .env.example
- Created ContactCategory enum with humanReadable method
- Implemented ContactSubmissionRequest form request
- Added ContactSubmission model with fillable and casts properties
- Included configurations for Discord alerts and honeypot protection

app/Models/ContactSubmission.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Model;
+use App\Enums\ContactCategory;
+
+class ContactSubmission extends Model
+{
+    protected $fillable
+        = [
+            'name',
+            'email',
+            'discord',
+            'category',
+            'subject',
+            'message',
+        ];
+
+    protected $casts = [
+        'category' => ContactCategory::class,
+    ];
+}