billing: Nebenläufigkeit & Idempotenz härten (H4, Stripe, Boost-Slots)

- CreditTopup-Gutschrift, MAN-Rechnung, Proof-PDF und Stripe-Webhook-
  Spiegelung sperren die betroffene Zeile innerhalb der Transaktion und
  prüfen die Idempotenz erst danach (verhindert Doppelgutschrift bei
  at-least-once-Webhooks, H4).
- Gemeinsamer LocksModelForUpdate-Trait für die häufige Lock-by-Key-Form
  (CreditTopup, ManualInvoice).
- Unique-Index auf invoices.stripe_invoice_id; Boost-Inventar über
  boost_slot_locks serialisiert, verwaiste Kategorie-Locks werden beim
  Löschen der Rubrik entfernt.

Deployment: vor migrate prüfen, ob invoices.stripe_invoice_id bereits
Duplikate enthält (die Unique-Migration bricht sonst bewusst ab).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

database/migrations/2026_07_14_125600_create_boost_slot_locks_table.php

@@ -0,0 +1,27 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::create('boost_slot_locks', function (Blueprint $table) {
+            $table->id();
+            $table->string('lock_key', 191)->unique();
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::dropIfExists('boost_slot_locks');
+    }
+};

database/migrations/2026_07_14_125700_add_unique_stripe_invoice_id_to_invoices_table.php

@@ -0,0 +1,24 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('invoices', function (Blueprint $table): void {
+            $table->dropIndex('invoices_stripe_invoice_idx');
+            $table->unique('stripe_invoice_id', 'invoices_stripe_invoice_unique');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('invoices', function (Blueprint $table): void {
+            $table->dropUnique('invoices_stripe_invoice_unique');
+            $table->index('stripe_invoice_id', 'invoices_stripe_invoice_idx');
+        });
+    }
+};