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/inventory/docs/data-model.md

@@ -0,0 +1,82 @@
+# Inventory Data Model
+
+Entity relationships and database schema for the inventory module.
+
+## Entity Relationship Overview
+
+```
+Store 1──* Inventory *──1 Product
+                │
+                └──* InventoryTransaction
+                         │
+                         └──? Order (for reserve/fulfill/release)
+```
+
+## Models
+
+### Inventory
+
+Stock quantities at warehouse bin locations. Supports multi-location inventory with reservation tracking.
+
+| Field | Type | Constraints | Description |
+|-------|------|-------------|-------------|
+| `id` | Integer | PK | Primary key |
+| `product_id` | Integer | FK, not null, indexed | Product reference |
+| `store_id` | Integer | FK, not null, indexed | Store reference |
+| `warehouse` | String | not null, default "strassen", indexed | Warehouse identifier |
+| `bin_location` | String | not null, indexed | Bin code (e.g., "SA-10-02") |
+| `quantity` | Integer | not null, default 0 | Total quantity at bin |
+| `reserved_quantity` | Integer | default 0 | Reserved/allocated quantity |
+| `gtin` | String | indexed | GTIN reference (duplicated for reporting) |
+| `created_at` | DateTime | tz-aware | Record creation time |
+| `updated_at` | DateTime | tz-aware | Record update time |
+
+**Unique Constraint**: `(product_id, warehouse, bin_location)`
+**Composite Indexes**: `(store_id, product_id)`, `(warehouse, bin_location)`
+
+**Key Property**: `available_quantity` = max(0, quantity - reserved_quantity)
+
+### InventoryTransaction
+
+Complete audit trail for all stock movements with before/after snapshots.
+
+| Field | Type | Constraints | Description |
+|-------|------|-------------|-------------|
+| `id` | Integer | PK | Primary key |
+| `store_id` | Integer | FK, not null, indexed | Store reference |
+| `product_id` | Integer | FK, not null, indexed | Product reference |
+| `inventory_id` | Integer | FK, nullable, indexed | Inventory record reference |
+| `transaction_type` | Enum | not null, indexed | Type of stock movement |
+| `quantity_change` | Integer | not null | Change amount (+ add, - remove) |
+| `quantity_after` | Integer | not null | Quantity snapshot after transaction |
+| `reserved_after` | Integer | not null, default 0 | Reserved quantity snapshot |
+| `location` | String | nullable | Location context |
+| `warehouse` | String | nullable | Warehouse context |
+| `order_id` | Integer | FK, nullable, indexed | Related order |
+| `order_number` | String | nullable | Order number for display |
+| `reason` | Text | nullable | Human-readable reason |
+| `created_by` | String | nullable | User/system identifier |
+| `created_at` | DateTime | not null, indexed | Timestamp (UTC) |
+
+**Composite Indexes**: `(store_id, product_id)`, `(store_id, created_at)`, `(transaction_type, created_at)`
+
+## Enums
+
+### TransactionType
+
+| Value | Description |
+|-------|-------------|
+| `reserve` | Stock reserved for order |
+| `fulfill` | Reserved stock consumed (shipped) |
+| `release` | Reserved stock released (cancelled) |
+| `adjust` | Manual adjustment (+/-) |
+| `set` | Set to exact quantity |
+| `import` | Initial import/sync |
+| `return` | Stock returned from customer |
+
+## Design Patterns
+
+- **Multi-location**: Inventory tracked per warehouse + bin location
+- **Reservation system**: Separate quantity and reserved_quantity for order holds
+- **Full audit trail**: Every stock change recorded with before/after snapshots
+- **Order integration**: Transactions linked to orders for reserve/fulfill/release cycle