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/inventory/docs/index.md

@@ -0,0 +1,53 @@
+# Inventory Management
+
+Stock level tracking, inventory locations, low stock alerts, transaction history, and bulk imports.
+
+## Overview
+
+| Aspect | Detail |
+|--------|--------|
+| Code | `inventory` |
+| Classification | Optional |
+| Dependencies | `catalog`, `orders` |
+| Status | Active |
+
+## Features
+
+- `inventory_basic` — Basic stock tracking
+- `inventory_locations` — Multi-location inventory
+- `low_stock_alerts` — Low stock notifications
+- `inventory_purchase_orders` — Purchase order management
+- `product_management` — Product inventory management
+- `inventory_transactions` — Stock movement audit trail
+- `inventory_import` — Bulk stock import
+
+## Permissions
+
+| Permission | Description |
+|------------|-------------|
+| `stock.view` | View inventory data |
+| `stock.edit` | Edit stock levels |
+| `stock.transfer` | Transfer stock between locations |
+
+## Data Model
+
+See [Data Model](data-model.md) for full entity relationships and schema.
+
+- **Inventory** — Stock quantities at warehouse bin locations
+- **InventoryTransaction** — Complete audit trail for stock movements
+
+## API Endpoints
+
+| Method | Path | Description |
+|--------|------|-------------|
+| `*` | `/api/v1/admin/inventory/*` | Admin inventory management |
+| `*` | `/api/v1/store/inventory/*` | Store inventory management |
+
+## Configuration
+
+No module-specific configuration.
+
+## Additional Documentation
+
+- [Data Model](data-model.md) — Entity relationships and database schema
+- [User Guide](user-guide.md) — Inventory management guide with API reference