feat: antigravity 配额域限流 + SSE 上限 (#222)

* fix: 添加 gemini-3-flash 前缀映射支持 gemini-3-flash-preview

* feat(antigravity): 增强请求参数和注入 Antigravity 身份 system prompt

* feat: antigravity 配额域限流

* chore: 调整 SSE 单行上限到 25MB

* chore: 提升 SSE 单行上限到 40MB

backend/internal/service/gemini_messages_compat_service.go

@@ -114,7 +114,7 @@ func (s *GeminiMessagesCompatService) SelectAccountForModelWithExclusions(ctx co
 			if _, excluded := excludedIDs[accountID]; !excluded {
 				account, err := s.accountRepo.GetByID(ctx, accountID)
 				// 检查账号是否有效：原生平台直接匹配，antigravity 需要启用混合调度
-				if err == nil && account.IsSchedulable() && (requestedModel == "" || s.isModelSupportedByAccount(account, requestedModel)) {
+				if err == nil && account.IsSchedulableForModel(requestedModel) && (requestedModel == "" || s.isModelSupportedByAccount(account, requestedModel)) {
 					valid := false
 					if account.Platform == platform {
 						valid = true
@@ -172,6 +172,9 @@ func (s *GeminiMessagesCompatService) SelectAccountForModelWithExclusions(ctx co
 		if useMixedScheduling && acc.Platform == PlatformAntigravity && !acc.IsMixedSchedulingEnabled() {
 			continue
 		}
+		if !acc.IsSchedulableForModel(requestedModel) {
+			continue
+		}
 		if requestedModel != "" && !s.isModelSupportedByAccount(acc, requestedModel) {
 			continue
 		}