refactor(upstream): replace upstream account type with apikey, auto-append /antigravity

Upstream accounts now use the standard APIKey type instead of a dedicated
upstream type. GetBaseURL() and new GetGeminiBaseURL() automatically append
/antigravity for Antigravity platform APIKey accounts, eliminating the need
for separate upstream forwarding methods.

- Remove ForwardUpstream, ForwardUpstreamGemini, testUpstreamConnection
- Remove upstream branch guards in Forward/ForwardGemini/TestConnection
- Add migration 052 to convert existing upstream accounts to apikey
- Update frontend CreateAccountModal to create apikey type
- Add unit tests for GetBaseURL and GetGeminiBaseURL

frontend/src/components/account/CreateAccountModal.vue

@@ -2289,9 +2289,9 @@ watch(
 watch(
   [accountCategory, addMethod, antigravityAccountType],
   ([category, method, agType]) => {
-    // Antigravity upstream 类型
+    // Antigravity upstream 类型（实际创建为 apikey）
     if (form.platform === 'antigravity' && agType === 'upstream') {
-      form.type = 'upstream'
+      form.type = 'apikey'
       return
     }
     if (category === 'oauth-based') {
@@ -2715,7 +2715,7 @@ const handleSubmit = async () => {
     submitting.value = true
     try {
       const extra = mixedScheduling.value ? { mixed_scheduling: true } : undefined
-      await createAccountAndFinish(form.platform, 'upstream', credentials, extra)
+      await createAccountAndFinish(form.platform, 'apikey', credentials, extra)
     } catch (error: any) {
       appStore.showError(error.response?.data?.detail || t('admin.accounts.failedToCreate'))
     } finally {