docs: migrate module documentation to single source of truth

Move 39 documentation files from top-level docs/ into each module's
docs/ folder, accessible via symlinks from docs/modules/. Create
data-model.md files for 10 modules with full schema documentation.
Replace originals with redirect stubs. Remove empty guide stubs.

Modules migrated: tenancy, billing, loyalty, marketplace, orders,
messaging, cms, catalog, inventory, hosting, prospecting.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/marketplace/docs/index.md

@@ -0,0 +1,73 @@
+# Marketplace (Letzshop)
+
+Letzshop marketplace integration for product sync, order import, and catalog synchronization.
+
+## Overview
+
+| Aspect | Detail |
+|--------|--------|
+| Code | `marketplace` |
+| Classification | Optional |
+| Dependencies | `inventory`, `catalog`, `orders` |
+| Status | Active |
+
+## Features
+
+- `letzshop_sync` — Letzshop API synchronization
+- `marketplace_import` — Product and order import
+- `product_sync` — Bidirectional product sync
+- `order_import` — Marketplace order import
+- `marketplace_analytics` — Marketplace performance metrics
+
+## Permissions
+
+| Permission | Description |
+|------------|-------------|
+| `marketplace.view_integration` | View marketplace integration |
+| `marketplace.manage_integration` | Manage marketplace settings |
+| `marketplace.sync_products` | Trigger product sync |
+
+## Data Model
+
+See [Data Model](data-model.md) for full entity relationships.
+
+- **MarketplaceProduct** — Canonical product data from marketplace sources
+- **MarketplaceProductTranslation** — Localized product content (title, description)
+- **MarketplaceImportJob** — CSV import job tracking with metrics
+- **MarketplaceImportError** — Detailed error records per import
+- **StoreLetzshopCredentials** — Encrypted API keys and sync settings
+- **LetzshopOrder** — Imported orders from Letzshop
+- **LetzshopFulfillmentQueue** — Outbound operation queue with retry logic
+- **LetzshopSyncLog** — Audit trail for sync operations
+- **LetzshopHistoricalImportJob** — Historical order import progress
+- **LetzshopStoreCache** — Marketplace store directory cache
+- **StoreOnboarding** — Store onboarding step tracking
+
+## API Endpoints
+
+| Method | Path | Description |
+|--------|------|-------------|
+| `*` | `/api/v1/admin/marketplace/*` | Admin marketplace management |
+| `*` | `/api/v1/admin/letzshop/*` | Letzshop-specific endpoints |
+| `*` | `/api/v1/admin/marketplace-products/*` | Product mapping management |
+
+## Scheduled Tasks
+
+| Task | Schedule | Description |
+|------|----------|-------------|
+| `marketplace.sync_store_directory` | Daily 02:00 | Sync store directory from Letzshop |
+
+## Configuration
+
+Letzshop API credentials are configured per-store via the admin UI.
+
+## Additional Documentation
+
+- [Data Model](data-model.md) — Entity relationships and database schema
+- [Architecture](architecture.md) — Multi-marketplace integration architecture
+- [Integration Guide](integration-guide.md) — CSV import guide with store/admin interfaces
+- [API Reference](api.md) — Letzshop GraphQL API reference
+- [Order Integration](order-integration.md) — Bidirectional order management with Letzshop
+- [Admin Guide](admin-guide.md) — Admin portal management guide
+- [Import Improvements](import-improvements.md) — GraphQL field mapping and EAN matching
+- [Job Queue](job-queue.md) — Job queue improvements and table harmonization