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/customers/docs/index.md

@@ -0,0 +1,47 @@
+# Customer Management
+
+Customer database, profiles, addresses, and segmentation.
+
+## Overview
+
+| Aspect | Detail |
+|--------|--------|
+| Code | `customers` |
+| Classification | Core |
+| Dependencies | None |
+| Status | Active |
+
+## Features
+
+- `customer_view` — View customer records
+- `customer_export` — Export customer data
+- `customer_profiles` — Customer profile management
+- `customer_segmentation` — Customer segmentation and filtering
+- `customer_addresses` — Address book management
+- `customer_authentication` — Storefront customer login/registration
+
+## Permissions
+
+| Permission | Description |
+|------------|-------------|
+| `customers.view` | View customer records |
+| `customers.edit` | Edit customer data |
+| `customers.delete` | Delete customers |
+| `customers.export` | Export customer data |
+
+## Data Model
+
+- **Customer** — Customer records with store-scoped profiles
+- **PasswordResetToken** — Password reset tokens for customer accounts
+
+## API Endpoints
+
+| Method | Path | Description |
+|--------|------|-------------|
+| `*` | `/api/v1/admin/customers/*` | Admin customer management |
+| `*` | `/api/v1/store/customers/*` | Store-level customer management |
+| `*` | `/api/v1/storefront/customers/*` | Customer self-service (profile, addresses) |
+
+## Configuration
+
+No module-specific configuration.