Add eBay Browse API integration with daily call quota

eBay marketplaces are now polled through eBay's official Buy > Browse API (client-credentials OAuth2) instead of an Apify scraper actor; Apify still handles Yahoo JP and Mercari. Browse API calls are tracked per day in a new ebay_api_usage table and capped (default 5000, configurable) on eBay's Pacific-time reset clock, so polling halts before the limit is hit. Credentials live in config.toml [ebay] and are overridable via /settings, which also surfaces the day's running call count.

Also carries the server.secure_cookies config plumbing (field, accessor, example) consumed by the following commit.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/ebay/ebay_test.go

@@ -0,0 +1,45 @@
+package ebay
+
+import "testing"
+
+func TestMarketplaceID(t *testing.T) {
+	cases := map[string]string{
+		"ebay.com":     "EBAY_US",
+		"ebay.co.uk":   "EBAY_GB",
+		"ebay.de":      "EBAY_DE",
+		"ebay.com.au":  "EBAY_AU",
+		"EBAY.CA":      "EBAY_CA",
+		"ebay":         "EBAY_US",
+		"weird-market": "EBAY_US",
+		" ebay.it ":    "EBAY_IT",
+	}
+	for in, want := range cases {
+		if got := MarketplaceID(in); got != want {
+			t.Errorf("MarketplaceID(%q) = %q, want %q", in, got, want)
+		}
+	}
+}
+
+func TestIsEbayMarketplace(t *testing.T) {
+	if !IsEbayMarketplace("ebay.co.uk") {
+		t.Error("ebay.co.uk should be an eBay marketplace")
+	}
+	if IsEbayMarketplace("yahoo-auctions-jp") {
+		t.Error("yahoo should not be an eBay marketplace")
+	}
+}
+
+func TestBuyingOptionsFilter(t *testing.T) {
+	cases := map[string]string{
+		"":           "",
+		"all":        "",
+		"bin":        "buyingOptions:{FIXED_PRICE}",
+		"buy_it_now": "buyingOptions:{FIXED_PRICE}",
+		"auction":    "buyingOptions:{AUCTION}",
+	}
+	for in, want := range cases {
+		if got := buyingOptionsFilter(in); got != want {
+			t.Errorf("buyingOptionsFilter(%q) = %q, want %q", in, got, want)
+		}
+	}
+}