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/Models/DeclarationInvitation.php

@@ -0,0 +1,74 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Support\Str;
+
+class DeclarationInvitation extends Model
+{
+    protected $table = 'declaration_invitations';
+
+    /**
+     * The attributes that are mass assignable.
+     *
+     * @var list<string>
+     */
+    protected $fillable = [
+        'declaration_id',
+        'token',
+        'email',
+        'expires_at',
+        'used_at',
+    ];
+
+    /**
+     * Get the attributes that should be cast.
+     *
+     * @return array<string, string>
+     */
+    protected function casts(): array
+    {
+        return [
+            'expires_at' => 'datetime',
+            'used_at' => 'datetime',
+        ];
+    }
+
+    /**
+     * Boot the model.
+     */
+    protected static function boot(): void
+    {
+        parent::boot();
+
+        static::creating(function (DeclarationInvitation $invitation) {
+            if (empty($invitation->token)) {
+                $invitation->token = Str::uuid()->toString();
+            }
+        });
+    }
+
+    /**
+     * Get the declaration that owns the invitation.
+     *
+     * @return BelongsTo<Declaration, $this>
+     */
+    public function declaration(): BelongsTo
+    {
+        return $this->belongsTo(Declaration::class);
+    }
+
+    /**
+     * Check if the invitation is valid (not expired, not used).
+     */
+    public function isValid(): bool
+    {
+        if ($this->used_at !== null) {
+            return false;
+        }
+
+        return $this->expires_at->isFuture();
+    }
+}