feat: add soft deletes to WorkspaceInvitation model

Invitations were hard-deleted when cancelled, removing the audit trail.
Add SoftDeletes trait and migration for deleted_at column so cancelled
invitations are retained for auditing purposes.

Fixes #22

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

Migrations/2026_03_24_000000_add_soft_deletes_to_workspace_invitations_table.php

@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Add soft deletes to workspace invitations for audit trail.
+     */
+    public function up(): void
+    {
+        Schema::table('workspace_invitations', function (Blueprint $table) {
+            $table->softDeletes();
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('workspace_invitations', function (Blueprint $table) {
+            $table->dropSoftDeletes();
+        });
+    }
+};