Complete Email Sortierer implementation with Appwrite and Stripe integration

.kiro/specs/email-sorter-setup/design.md

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+# Design Document
+
+## Overview
+
+Das Email-Sortierer System besteht aus drei Hauptkomponenten:
+1. Frontend: Vanilla JavaScript Multi-Step-Formular
+2. Backend: Express.js Server mit API-Endpunkten
+3. Datenbank: Appwrite Collections für Produkte, Fragen, Submissions, Antworten und Orders
+
+## Architecture
+
+```
+Browser (HTML/JS) 
+    ↓ HTTP
+Express Server
+    ↓ API Calls
+Appwrite Database
+    
+Express Server
+    ↓ Webhook
+Stripe Payment
+```
+
+## Components and Interfaces
+
+### Frontend (public/index.html)
+- Multi-step form renderer
+- State management für Antworten
+- API calls zu Backend
+
+### Backend (server/index.mjs)
+- GET /api/questions - Lädt Fragen aus Appwrite
+- POST /api/submissions - Speichert Kundenantworten
+- POST /api/checkout - Erstellt Stripe Checkout Session
+- POST /stripe/webhook - Empfängt Stripe Events
+
+### Bootstrap Script (server/bootstrap-appwrite.mjs)
+- Erstellt Appwrite Database und Collections
+- Erstellt alle Attribute/Spalten
+- Seeded Produkt und 13 Fragen
+
+## Data Models
+
+### Products Collection
+- slug: string
+- title: string
+- description: string
+- priceCents: integer
+- currency: string
+- isActive: boolean
+
+### Questions Collection
+- productId: string
+- key: string
+- label: string
+- helpText: string (optional)
+- type: string (text, email, select, multiselect, textarea)
+- required: boolean
+- step: integer
+- order: integer
+- optionsJson: string (optional)
+- isActive: boolean
+
+### Submissions Collection
+- productId: string
+- status: string (draft, paid)
+- customerEmail: email (optional)
+- customerName: string (optional)
+- utmJson: string (optional)
+- finalSummaryJson: string
+- priceCents: integer
+- currency: string
+
+### Answers Collection
+- submissionId: string
+- answersJson: string
+
+### Orders Collection
+- submissionId: string
+- orderDataJson: string
+
+## Correctness Properties
+
+*Properties sind formale Aussagen über das Systemverhalten, die über alle gültigen Eingaben gelten.*
+
+**Property 1: Question Loading**
+*For any* active product, when questions are requested, all active questions for that product should be returned ordered by step and order.
+**Validates: Requirements 1.1, 2.4**
+
+**Property 2: Submission Creation**
+*For any* valid answers object, when a submission is created, the system should store the submission and return a valid submissionId.
+**Validates: Requirements 2.2, 2.3**
+
+**Property 3: Payment Flow**
+*For any* valid submissionId, when checkout is initiated, the system should create a Stripe session and return a checkout URL.
+**Validates: Requirements 3.1, 3.2**
+
+**Property 4: Webhook Validation**
+*For any* Stripe webhook event, when the signature is invalid, the system should reject the request with 400 status.
+**Validates: Requirements 3.4**
+
+## Error Handling
+
+- Fehlende Umgebungsvariablen → Server exit mit Fehler
+- Ungültige Webhook-Signatur → 400 Bad Request
+- Fehlende submissionId → 400 Bad Request
+- Appwrite Fehler → Console error + graceful handling
+
+## Testing Strategy
+
+**Unit Tests:**
+- API endpoint responses
+- Data validation
+- Error handling
+
+**Property-Based Tests:**
+- Question ordering across random datasets
+- Submission creation with various answer formats
+- Webhook signature validation
+
+Minimum 100 Iterationen pro Property Test.