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

backend/internal/service/account.go

@@ -425,6 +425,22 @@ func (a *Account) GetBaseURL() string {
 	if baseURL == "" {
 		return "https://api.anthropic.com"
 	}
+	if a.Platform == PlatformAntigravity {
+		return strings.TrimRight(baseURL, "/") + "/antigravity"
+	}
+	return baseURL
+}
+
+// GetGeminiBaseURL 返回 Gemini 兼容端点的 base URL。
+// Antigravity 平台的 APIKey 账号自动拼接 /antigravity。
+func (a *Account) GetGeminiBaseURL(defaultBaseURL string) string {
+	baseURL := strings.TrimSpace(a.GetCredential("base_url"))
+	if baseURL == "" {
+		return defaultBaseURL
+	}
+	if a.Platform == PlatformAntigravity && a.Type == AccountTypeAPIKey {
+		return strings.TrimRight(baseURL, "/") + "/antigravity"
+	}
 	return baseURL
 }