Credit-Wallet + Ledger + Tier-Preisableitung (Fundament)

Echte Credit-Wallet (1 Credit = 1 EUR) mit append-only Ledger als Basis fuer
die Credit-Oekonomie aus dem Decision-Update (Rev. 4):

- credit_wallets (denormalisierter Saldo) + credit_transactions (Ledger,
  vorzeichenbehaftet, balance_after, polymorphe reference)
- CreditWalletService: einziger Schreibpfad, atomar mit Row-Lock,
  InsufficientCreditsException mit shortfall fuer den Mini-Checkout
- Tier-Enum (Einzel/Starter/Business/Pro/Agency) + User::currentTier()
- CreditPricingService: tier-gestaffelte Ableitung aus config/credits.php
  (Extra-PM 19/15/12/10/8, Boost 12/20/35, PDF 3, Depublish 25, Pruef-Quota)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/Feature/CreditWalletServiceTest.php

@@ -0,0 +1,67 @@
+<?php
+
+use App\Enums\CreditTransactionType;
+use App\Exceptions\InsufficientCreditsException;
+use App\Models\User;
+use App\Services\Billing\CreditWalletService;
+
+beforeEach(function (): void {
+    $this->service = app(CreditWalletService::class);
+});
+
+test('a credit raises the balance and writes a ledger row', function () {
+    $user = User::factory()->create();
+
+    $tx = $this->service->credit($user, 20, CreditTransactionType::Topup, 'Paket 20');
+
+    expect($this->service->balance($user))->toBe(20);
+    expect($tx->amount_credits)->toBe(20);
+    expect($tx->balance_after)->toBe(20);
+    expect($tx->type)->toBe(CreditTransactionType::Topup);
+});
+
+test('a debit lowers the balance and records the balance after', function () {
+    $user = User::factory()->create();
+    $this->service->credit($user, 20);
+
+    $tx = $this->service->debit($user, 12, 'Extra-PM');
+
+    expect($this->service->balance($user))->toBe(8);
+    expect($tx->amount_credits)->toBe(-12);
+    expect($tx->balance_after)->toBe(8);
+    expect($tx->type)->toBe(CreditTransactionType::Spend);
+});
+
+test('debiting more than the balance throws and keeps the balance untouched', function () {
+    $user = User::factory()->create();
+    $this->service->credit($user, 8);
+
+    $threw = null;
+    try {
+        $this->service->debit($user, 15);
+    } catch (InsufficientCreditsException $e) {
+        $threw = $e;
+    }
+
+    expect($threw)->toBeInstanceOf(InsufficientCreditsException::class);
+    expect($threw->shortfall())->toBe(7);
+    expect($this->service->balance($user))->toBe(8);
+    expect($user->creditTransactions()->count())->toBe(1); // nur die Aufladung
+});
+
+test('balance is zero and canAfford is false without a wallet', function () {
+    $user = User::factory()->create();
+
+    expect($this->service->balance($user))->toBe(0);
+    expect($this->service->canAfford($user, 1))->toBeFalse();
+});
+
+test('a refund credits back a prior spend', function () {
+    $user = User::factory()->create();
+    $this->service->credit($user, 30);
+    $this->service->debit($user, 25, 'Depublizieren');
+
+    $this->service->credit($user, 25, CreditTransactionType::Refund, 'Widerruf Depublizieren');
+
+    expect($this->service->balance($user))->toBe(30);
+});