Merge upstream/main: v0.1.62-v0.1.64 updates

Major updates:
- fix(gateway): fix SSE streaming usage data overwrite issue
- feat(purchase): add purchase subscription iframe page and config
- fix(scheduler): fix race condition from empty cache
- feat(settings): add PurchaseSubscriptionEnabled and PurchaseSubscriptionURL

Resolved conflicts:
- backend/cmd/server/VERSION: updated to 0.1.61
- setting_service.go: keep TianShuAPI site name while adding purchase subscription features

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/internal/server/api_contract_test.go

@@ -489,7 +489,9 @@ func TestAPIContracts(t *testing.T) {
 					"enable_identity_patch": true,
 					"identity_patch_prompt": "",
 					"home_content": "",
-					"hide_ccs_import_button": false
+					"hide_ccs_import_button": false,
+					"purchase_subscription_enabled": false,
+					"purchase_subscription_url": ""
 				}
 			}`,
 		},