feat: complete Epic 0 — foundation migration & infrastructure setup

Stories 0.2-0.5: rename folders→declarations (backend+frontend), configure
Redis for cache/queue/sessions, add foundation database migrations
(permissions, archived_at), replace DeclarationStatus enum with architecture
lifecycle values, create DeclarationObserver for status transition validation
and auto-archive, fix controller status transitions to respect observer rules.

93 tests pass (240 assertions).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/Http/Requests/StoreDeclarationMentionRequest.php

@@ -0,0 +1,37 @@
+<?php
+
+namespace App\Http\Requests;
+
+use Illuminate\Foundation\Http\FormRequest;
+use Illuminate\Validation\Rule;
+
+class StoreDeclarationMentionRequest extends FormRequest
+{
+    /**
+     * Determine if the user is authorized to make this request.
+     */
+    public function authorize(): bool
+    {
+        return $this->user() !== null;
+    }
+
+    /**
+     * Get the validation rules that apply to the request.
+     *
+     * @return array<string, \Illuminate\Contracts\Validation\ValidationRule|array<mixed>|string>
+     */
+    public function rules(): array
+    {
+        $workspaceId = $this->session()->get('current_workspace_id');
+
+        return [
+            'user_id' => [
+                'required',
+                'integer',
+                Rule::exists('workspace_user', 'user_id')
+                    ->where('workspace_id', $workspaceId),
+            ],
+            'message' => ['required', 'string', 'max:500'],
+        ];
+    }
+}