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/handler/gateway_handler.go

@@ -482,7 +482,7 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 			if switchCount > 0 {
 				requestCtx = context.WithValue(requestCtx, ctxkey.AccountSwitchCount, switchCount)
 			}
-			if account.Platform == service.PlatformAntigravity {
+			if account.Platform == service.PlatformAntigravity && account.Type != service.AccountTypeAPIKey {
 				result, err = h.antigravityGatewayService.Forward(requestCtx, c, account, body, hasBoundSession)
 			} else {
 				result, err = h.gatewayService.Forward(requestCtx, c, account, parsedReq)

backend/internal/handler/gemini_v1beta_handler.go

@@ -410,7 +410,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 		if switchCount > 0 {
 			requestCtx = context.WithValue(requestCtx, ctxkey.AccountSwitchCount, switchCount)
 		}
-		if account.Platform == service.PlatformAntigravity {
+		if account.Platform == service.PlatformAntigravity && account.Type != service.AccountTypeAPIKey {
 			result, err = h.antigravityGatewayService.ForwardGemini(requestCtx, c, account, modelName, action, stream, body, hasBoundSession)
 		} else {
 			result, err = h.geminiCompatService.ForwardNative(requestCtx, c, account, modelName, action, stream, body)