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/contracts/docs/index.md

@@ -0,0 +1,33 @@
+# Module Contracts
+
+Cross-module contracts using Protocol pattern for type-safe inter-module communication.
+
+## Overview
+
+| Aspect | Detail |
+|--------|--------|
+| Code | `contracts` |
+| Classification | Core |
+| Dependencies | None |
+| Status | Active |
+
+## Features
+
+- `service_protocols` — Protocol definitions for cross-module service interfaces
+- `cross_module_interfaces` — Type-safe inter-module communication contracts
+
+## Permissions
+
+No permissions defined.
+
+## Data Model
+
+No database models.
+
+## API Endpoints
+
+No API endpoints.
+
+## Configuration
+
+No module-specific configuration.