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>

app/Models/CreditTransaction.php

@@ -0,0 +1,55 @@
+<?php
+
+namespace App\Models;
+
+use App\Enums\CreditTransactionType;
+use Database\Factories\CreditTransactionFactory;
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\MorphTo;
+
+/**
+ * Eine Zeile im Wallet-Ledger. Append-only: bestehende Buchungen werden nie
+ * verändert, Korrekturen erfolgen als Gegenbuchung (Refund).
+ */
+class CreditTransaction extends Model
+{
+    /** @use HasFactory<CreditTransactionFactory> */
+    use HasFactory;
+
+    protected $fillable = [
+        'credit_wallet_id',
+        'user_id',
+        'amount_credits',
+        'balance_after',
+        'type',
+        'description',
+        'reference_type',
+        'reference_id',
+    ];
+
+    protected function casts(): array
+    {
+        return [
+            'amount_credits' => 'integer',
+            'balance_after' => 'integer',
+            'type' => CreditTransactionType::class,
+        ];
+    }
+
+    public function wallet(): BelongsTo
+    {
+        return $this->belongsTo(CreditWallet::class, 'credit_wallet_id');
+    }
+
+    public function user(): BelongsTo
+    {
+        return $this->belongsTo(User::class);
+    }
+
+    public function reference(): MorphTo
+    {
+        return $this->morphTo();
+    }
+}