Add store loan methods with public website calculate API.

Supports dashboard CRUD and tenant list/get/calculate so storefronts can offer installments, requiring a method pick when several exist.

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/PROJECT_CONTEXT.md

@@ -214,6 +214,8 @@ StoreItem 1──* StoreItemVariant (purchasable SKUs: price, stock, variation c
 StoreItemVariant 1──* StoreItemVariantSelection → CategoryVariationOption
 Product 1──* ProductTechnicalFieldValue → CategoryTechnicalFormField
 
+Business 1──* StoreLoanMethod (installments / facilities; module `store_installments`)
+
 UserProduct *── Category (via CategoryAssignment, entityType user_product → product categories)
 UserProduct 1──* UserProductTechnicalFieldValue → CategoryTechnicalFormField
 UserProduct → City (country, city, optional district)
@@ -284,6 +286,9 @@ All routes are prefixed with `/api/v1`.
 | GET | `/tenants/:host/portfolios/by-id/:portfolioId` | Public portfolio by id |
 | POST | `/businesses/:id/website/sitemap/sync` | Import manifest from `GET https://{domain}/meshkee/sitemap-config.json` |
 | GET | `/tenants/:host/store-specials` | Active store specials (`source` is `product` or `store_item`) |
+| GET | `/tenants/:host/store-loan-methods` | Active installment/credit methods (`enabled` + `items[]`; needs `store` + `store_installments`) |
+| GET | `/tenants/:host/store-loan-methods/:methodId` | One active method |
+| POST | `/tenants/:host/store-loan-methods/calculate` | Installment plan — body requires `methodId` (pick method first if multiple) |
 | GET | `/tenants/:host/website/category-groups` | Homepage category rows |
 | GET | `/tenants/:host/website/brand-groups` | Homepage brand rows |
 | GET | `/tenants/:host/website/sliders` | Homepage sliders with slides |
@@ -518,6 +523,42 @@ Stored in `businesses.settings.website.specialProductsSource` (`product` | `stor
 - Dashboard: `PATCH /businesses/:id/settings` with `{ website: { specialProductsSource } }`.
 - Public: `GET /tenants/:host` includes `specialProductsSource`; `GET /tenants/:host/store-specials` includes top-level `source`.
 
+### Store loan methods (installments & facilities)
+
+Requires modules `store` + `store_installments` (opt-in). Dashboard: Store → اقساط و تسهیلات.
+
+| Field | Notes |
+|-------|-------|
+| `nameFa` / `nameEn` | Display names |
+| `minShoppingAmount` | Minimum purchase total to use this method |
+| `minAmount` / `maxAmount` | Allowed credit range |
+| `interest` | Flat annual %; fee = credit × (interest/100) × (months/12) |
+| `returnMonthsMin` / `Max` / `Step` | Term options (step ∈ 2,3,4,6,12) |
+| `imageMediaId` / `imageAspectRatio` | Optional image (`1:1` or `3:2`) |
+
+**Dashboard (auth, `products.read` / `products.update`):**
+- `GET/POST /businesses/:businessId/store-loan-methods`
+- `GET/PATCH/DELETE /businesses/:businessId/store-loan-methods/:methodId`
+
+**Public website:**
+- `GET /tenants/:host/store-loan-methods` → `{ enabled, items[] }` (`enabled: false` + empty when module off)
+- `GET /tenants/:host/store-loan-methods/:methodId` → `{ method }` (active only)
+- `POST /tenants/:host/store-loan-methods/calculate` → `{ method, plan }` — **always requires `methodId`**
+
+Calculator UX when a site has multiple methods: list first → if `items.length > 1`, ask the shopper which method → then collect purchase total / credit / months → `POST .../calculate`. If exactly one method, auto-select it. Never calculate without a chosen `methodId`.
+
+```json
+POST /tenants/:host/store-loan-methods/calculate
+{
+  "methodId": "1",
+  "totalValue": 5000000,
+  "creditAmount": 2000000,
+  "months": 12
+}
+```
+
+`plan` includes `cashPayment`, `interestFee`, `totalRepay`, and monthly `installments[]` (`index`, `dueDate`, `amount`). Public items also include `returnMonthOptions[]`.
+
 **Example — batch create variants for a product:**
 
 ```json
@@ -667,6 +708,7 @@ See `.env.example` for the full list. Key groups:
 - Products CRUD
 - Product variation values (which options a product offers)
 - Store items / product variants (price, stock, SKU)
+- Store loan methods / installments (`store_installments`): dashboard CRUD + public list/get/calculate
 - Torob Product API v3 (`POST /tenants/:host/torob_api/v3/products`) for store-module websites; nginx proxies `/torob_api/v3/products` on the shop apex
 - Shopping cart + checkout + orders (customer + admin)
 - Online e-payment (Mellat + ZarinPal; stubs for SEP / Snapp Pay / DigiPay)