feat(imports): add real pricing and subscription collectors

Add plan catalog and subscription schema support, seed baselines, and real importers for core domestic subscriptions plus stable official pricing sources.

This commit also hardens the shared fetch layers so the importers can support live collection and database writes instead of relying on manual placeholders alone.

scripts/import_catalog_seed_verification_test.go

@@ -0,0 +1,11 @@
+//go:build llm_script
+
+package main
+
+import "testing"
+
+func TestCatalogSeedVerificationImporterKeyConstant(t *testing.T) {
+	if catalogSeedVerificationImporterKey != "import_catalog_seed_verification.go" {
+		t.Fatalf("importer key 常量错误: %q", catalogSeedVerificationImporterKey)
+	}
+}