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/messaging/docs/index.md

@@ -0,0 +1,63 @@
+# Messaging & Notifications
+
+Core email and notification system for user registration, password resets, team invitations, and system notifications. Required for basic platform operations.
+
+## Overview
+
+| Aspect | Detail |
+|--------|--------|
+| Code | `messaging` |
+| Classification | Core |
+| Dependencies | None |
+| Status | Active |
+
+## Features
+
+- `customer_messaging` — Customer-facing email communications
+- `internal_messages` — Internal messaging system
+- `notification_center` — Admin notification center
+- `message_attachments` — File attachments for messages
+- `admin_notifications` — System notifications for admins
+
+## Permissions
+
+| Permission | Description |
+|------------|-------------|
+| `messaging.view_messages` | View messages |
+| `messaging.send_messages` | Send messages |
+| `messaging.manage_templates` | Manage email templates |
+
+## Data Model
+
+See [Data Model](data-model.md) for full entity relationships and schema.
+
+- **EmailTemplate** — Multi-language email templates with Jinja2 variables
+- **StoreEmailTemplate** — Per-store template overrides
+- **EmailLog** — Email sending history and delivery tracking
+- **StoreEmailSettings** — Per-store email provider configuration
+- **AdminNotification** — System alerts and admin notifications
+- **Conversation** — Threaded conversations with polymorphic participants
+- **ConversationParticipant** — Participant links (admin, store, customer)
+- **Message** — Individual messages with soft delete
+- **MessageAttachment** — File attachments for messages
+
+## API Endpoints
+
+| Method | Path | Description |
+|--------|------|-------------|
+| `*` | `/api/v1/admin/messages/*` | Message management |
+| `*` | `/api/v1/admin/notifications/*` | Notification management |
+| `*` | `/api/v1/admin/email-templates/*` | Email template CRUD |
+
+## Configuration
+
+Email provider settings are configured at the store level via the settings UI.
+
+## Additional Documentation
+
+- [Data Model](data-model.md) — Entity relationships and database schema
+- [Architecture](architecture.md) — Messaging system architecture
+- [Notifications](notifications.md) — Admin notification system
+- [Email System](email-system.md) — Email delivery system overview
+- [Email Settings](email-settings.md) — Provider configuration guide
+- [Email Settings Implementation](email-settings-impl.md) — Email settings technical details