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/catalog/docs/index.md

@@ -0,0 +1,57 @@
+# Product Catalog
+
+Product catalog browsing and search for storefronts.
+
+## Overview
+
+| Aspect | Detail |
+|--------|--------|
+| Code | `catalog` |
+| Classification | Optional |
+| Dependencies | None |
+| Status | Active |
+
+## Features
+
+- `product_catalog` — Product catalog browsing
+- `product_search` — Product search and filtering
+- `product_variants` — Product variant management
+- `product_categories` — Category hierarchy
+- `product_attributes` — Custom product attributes
+- `product_import_export` — Bulk product import/export
+
+## Permissions
+
+| Permission | Description |
+|------------|-------------|
+| `products.view` | View products |
+| `products.create` | Create products |
+| `products.edit` | Edit products |
+| `products.delete` | Delete products |
+| `products.import` | Import products |
+| `products.export` | Export products |
+
+## Data Model
+
+See [Data Model](data-model.md) for full entity relationships and schema.
+
+- **Product** — Store-specific product with pricing, VAT, and supplier fields
+- **ProductTranslation** — Multilingual content with SEO fields
+- **ProductMedia** — Product-media associations with usage types
+
+## API Endpoints
+
+| Method | Path | Description |
+|--------|------|-------------|
+| `*` | `/api/v1/admin/catalog/*` | Admin product management |
+| `*` | `/api/v1/store/catalog/*` | Store product management |
+| `GET` | `/api/v1/storefront/catalog/*` | Public product browsing |
+
+## Configuration
+
+No module-specific configuration.
+
+## Additional Documentation
+
+- [Data Model](data-model.md) — Entity relationships and database schema
+- [Architecture](architecture.md) — Independent product copy pattern and API design