feat: add pricing provider boundary

docs/API.md

@@ -89,6 +89,7 @@ POST   /api/v1/admin/sales/:id/void
 POST   /api/v1/admin/pricing/observations
 POST   /api/v1/admin/inventory/:id/valuation
 POST   /api/v1/admin/inventory/:id/valuation/override
+POST   /api/v1/admin/inventory/:id/pricing/refresh
 POST   /api/v1/admin/bulk
 
 GET    /api/v1/admin/dashboard

docs/PRICING.md

@@ -22,6 +22,12 @@ A provider returns normalized observations containing:
 
 Provider adapters own provider-specific API behavior and normalization.
 
+The application uses a validated provider registry. A refresh request selects a
+registered provider, normalizes its observation array through the existing
+pricing service, and recalculates the item valuation. Missing providers return
+an explicit unavailable response; the application never fabricates observations
+or values.
+
 Use officially supported APIs or permitted sources. Do not invent APIs or bypass restrictions.
 
 ## Provider Product Mapping