feat: add pricing observation API

Add seller-owned pricing observation persistence, median market valuation calculation, admin routes, contracts, validation, migration, and focused tests.

packages/contracts/package.json

@@ -15,5 +15,6 @@
     "./public-inventory": "./public-inventory.js",
     "./public-product": "./public-product.js",
     "./public-seller": "./public-seller.js"
+    ,"./pricing": "./pricing.js"
   }
 }

packages/contracts/pricing.js

@@ -0,0 +1,31 @@
+function toIsoString(value) { return value instanceof Date ? value.toISOString() : value; }
+
+export function toAdminPricingObservationDto(observation) {
+  return {
+    id: observation.id,
+    productId: observation.productId,
+    inventoryItemId: observation.inventoryItemId,
+    providerId: observation.providerId,
+    priceMinor: observation.priceMinor,
+    currency: observation.currency,
+    sourceIdentifier: observation.sourceIdentifier,
+    conditionId: observation.conditionId,
+    sourceUrl: observation.sourceUrl ?? null,
+    qualityScore: observation.qualityScore ?? null,
+    observedAt: toIsoString(observation.observedAt),
+  };
+}
+
+export function toAdminMarketValuationDto({ valuation, observations }) {
+  return {
+    id: valuation.id,
+    inventoryItemId: valuation.inventoryItemId,
+    valueMinor: valuation.valueMinor,
+    currency: valuation.currency,
+    strategy: valuation.strategy,
+    confidence: valuation.confidence,
+    sourceCount: valuation.sourceCount,
+    calculatedAt: toIsoString(valuation.calculatedAt),
+    observations: observations.map(toAdminPricingObservationDto),
+  };
+}