# 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.