v2.0: massive test data (104 products, 41 postings, 20 returns, 20 payouts, 25 txs, 3 companies)

- Remove unused gin dependency from go.mod
- Remove credential logging from startup
- Remove dead parseOfferIDs function
- Add method check to GET /v1/product/list
- Complete all filter implementations (since, to, status, product_id, return_id, dir, analytics, page)
- CancelPosting now validates posting existence (404 for unknown)
- Implement MOCK_STOCKS_JSON file loading
- Add Phase 2 endpoints: v3/product/import, v1/product/info/description,
  v1/product/info/attributes, v2/category/tree, v3/category/attribute,
  v3/category/attribute/values
- Generate 104 products/stocks/prices across 3 companies:
  TechGiant (TG-0001..TG-0035), FashionLine (FL-0001..FL-0035), HomeStyle (HS-0001..HS-0034)
- 41 postings, 20 returns, 20 payouts, 25 transactions
- 31 tests (all PASS with race detector)
- Add Makefile

Makefile

@@ -0,0 +1,25 @@
+.PHONY: build test docker run clean vet lint
+
+build:
+	CGO_ENABLED=0 go build -o ozon-mock ./cmd/server
+
+test:
+	go test -v -race ./...
+
+vet:
+	go vet ./...
+
+lint:
+	go vet ./...
+
+run:
+	go run ./cmd/server
+
+docker:
+	CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -o ozon-mock ./cmd/server
+	docker build -t ozon-mock .
+
+clean:
+	rm -f ozon-mock
+
+all: vet test build