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/billing/docs/index.md

@@ -0,0 +1,74 @@
+# Billing & Subscriptions
+
+Core subscription management, tier limits, store billing, and invoice history. Provides tier-based feature gating used throughout the platform. Uses the payments module for actual payment processing.
+
+## Overview
+
+| Aspect | Detail |
+|--------|--------|
+| Code | `billing` |
+| Classification | Core |
+| Dependencies | `payments` |
+| Status | Active |
+
+## Features
+
+- `subscription_management` — Subscription lifecycle management
+- `billing_history` — Billing and payment history
+- `invoice_generation` — Automatic invoice generation
+- `subscription_analytics` — Subscription metrics and analytics
+- `trial_management` — Free trial period management
+- `limit_overrides` — Per-store tier limit overrides
+
+## Permissions
+
+| Permission | Description |
+|------------|-------------|
+| `billing.view_tiers` | View subscription tiers |
+| `billing.manage_tiers` | Manage subscription tiers |
+| `billing.view_subscriptions` | View subscriptions |
+| `billing.manage_subscriptions` | Manage subscriptions |
+| `billing.view_invoices` | View invoices |
+
+## Data Model
+
+See [Data Model](data-model.md) for full entity relationships.
+
+- **SubscriptionTier** — Tier definitions with Stripe price IDs
+- **TierFeatureLimit** — Per-tier feature limits (feature_code + limit_value)
+- **MerchantSubscription** — Per-merchant+platform subscription state
+- **MerchantFeatureOverride** — Per-merchant feature limit overrides
+- **AddOnProduct / StoreAddOn** — Purchasable add-ons
+- **BillingHistory** — Invoice and payment records
+- **StripeWebhookEvent** — Webhook idempotency tracking
+
+## API Endpoints
+
+| Method | Path | Description |
+|--------|------|-------------|
+| `*` | `/api/v1/admin/billing/*` | Admin billing management |
+| `*` | `/api/v1/admin/features/*` | Feature/tier management |
+| `*` | `/api/v1/merchant/billing/*` | Merchant billing endpoints |
+| `*` | `/api/v1/platform/billing/*` | Platform-wide billing stats |
+
+## Scheduled Tasks
+
+| Task | Schedule | Description |
+|------|----------|-------------|
+| `billing.reset_period_counters` | Daily 00:05 | Reset period-based usage counters |
+| `billing.check_trial_expirations` | Daily 01:00 | Check and handle expired trials |
+| `billing.sync_stripe_status` | Hourly :30 | Sync subscription status with Stripe |
+| `billing.cleanup_stale_subscriptions` | Weekly Sunday 03:00 | Clean up stale subscription records |
+
+## Configuration
+
+Configured via Stripe environment variables and tier definitions in the admin panel.
+
+## Additional Documentation
+
+- [Data Model](data-model.md) — Entity relationships and database schema
+- [Subscription System](subscription-system.md) — Architecture, feature providers, API reference
+- [Feature Gating](feature-gating.md) — Tier-based feature access control and UI integration
+- [Tier Management](tier-management.md) — Admin guide for managing subscription tiers
+- [Subscription Workflow](subscription-workflow.md) — Subscription lifecycle and implementation phases
+- [Stripe Integration](stripe-integration.md) — Stripe Connect setup, webhooks, payment flow