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/monitoring/docs/index.md

@@ -0,0 +1,44 @@
+# Platform Monitoring
+
+Logs, background tasks, imports, system health, Flower, and Grafana integration.
+
+## Overview
+
+| Aspect | Detail |
+|--------|--------|
+| Code | `monitoring` |
+| Classification | Internal |
+| Dependencies | None |
+| Status | Active |
+
+## Features
+
+- `application_logs` — Application log viewer
+- `background_tasks` — Celery task monitoring
+- `import_jobs` — Import job tracking
+- `capacity_monitoring` — System capacity metrics
+- `testing_hub` — Test execution monitoring
+- `code_quality` — Code quality dashboard
+- `flower_integration` — Flower (Celery monitoring) integration
+- `grafana_integration` — Grafana dashboard integration
+
+## Permissions
+
+No permissions defined (internal module, admin-only access).
+
+## Data Model
+
+- **CapacitySnapshot** — System capacity metric snapshots
+
+## API Endpoints
+
+| Method | Path | Description |
+|--------|------|-------------|
+| `*` | `/api/v1/admin/logs/*` | Log viewer |
+| `*` | `/api/v1/admin/platform-health/*` | Platform health metrics |
+| `*` | `/api/v1/admin/audit/*` | Audit log viewer |
+| `*` | `/api/v1/admin/code-quality/*` | Code quality dashboard |
+
+## Configuration
+
+No module-specific configuration.