feat: add pricing quality controls

Add robust outlier detection, stale valuation metadata, and audited seller-scoped market-value overrides with migration and tests.

database/migrations/0016_pricing_quality.sql

@@ -0,0 +1,9 @@
+CREATE TABLE "market_value_overrides" (
+  "id" uuid PRIMARY KEY DEFAULT gen_random_uuid() NOT NULL,
+  "inventory_item_id" uuid NOT NULL REFERENCES "inventory_items"("id"),
+  "value_minor" integer NOT NULL,
+  "currency" text NOT NULL,
+  "reason" text NOT NULL,
+  "created_by" uuid NOT NULL REFERENCES "users"("id"),
+  "created_at" timestamp with time zone DEFAULT now() NOT NULL
+);