fix: resolve 5 audit findings in channel/credits/scheduling

P0-1: Credits degraded response retry + fail-open
- Add isAntigravityDegradedResponse() to detect transient API failures
- Retry up to 3 times with exponential backoff (500ms/1s/2s)
- Invalidate singleflight cache between retries
- Fail-open after exhausting retries instead of 5h circuit break

P1-1: Fix channel restriction pre-check timing conflict
- Swap checkClaudeCodeRestriction before checkChannelPricingRestriction
- Ensures channel restriction is checked against final fallback groupID

P1-2: Add interval pricing validation (frontend + backend)
- Backend: ValidateIntervals() with boundary, price, overlap checks
- Frontend: validateIntervals() with Chinese error messages
- Rules: MinTokens>=0, MaxTokens>MinTokens, prices>=0, no overlap

P2: Fix cross-platform same-model pricing/mapping override
- Store cache keys using original platform instead of group platform
- Lookup across matching platforms (antigravity→anthropic→gemini)
- Prevents anthropic/gemini same-name models from overwriting each other

backend/internal/service/gateway_service.go

@@ -1178,11 +1178,6 @@ func (s *GatewayService) SelectAccountForModel(ctx context.Context, groupID *int
 
 // SelectAccountForModelWithExclusions selects an account supporting the requested model while excluding specified accounts.
 func (s *GatewayService) SelectAccountForModelWithExclusions(ctx context.Context, groupID *int64, sessionHash string, requestedModel string, excludedIDs map[int64]struct{}) (*Account, error) {
-	// 渠道定价限制预检查（requested / channel_mapped 基准）
-	if s.checkChannelPricingRestriction(ctx, groupID, requestedModel) {
-		return nil, fmt.Errorf("%w supporting model: %s (channel pricing restriction)", ErrNoAvailableAccounts, requestedModel)
-	}
-
 	// 优先检查 context 中的强制平台（/antigravity 路由）
 	var platform string
 	forcePlatform, hasForcePlatform := ctx.Value(ctxkey.ForcePlatform).(string)
@@ -1201,6 +1196,12 @@ func (s *GatewayService) SelectAccountForModelWithExclusions(ctx context.Context
 		platform = PlatformAnthropic
 	}
 
+	// Claude Code 限制可能已将 groupID 解析为 fallback group，
+	// 渠道限制预检查必须使用解析后的分组。
+	if s.checkChannelPricingRestriction(ctx, groupID, requestedModel) {
+		return nil, fmt.Errorf("%w supporting model: %s (channel pricing restriction)", ErrNoAvailableAccounts, requestedModel)
+	}
+
 	// anthropic/gemini 分组支持混合调度（包含启用了 mixed_scheduling 的 antigravity 账户）
 	// 注意：强制平台模式不走混合调度
 	if (platform == PlatformAnthropic || platform == PlatformGemini) && !hasForcePlatform {
@@ -1217,11 +1218,6 @@ func (s *GatewayService) SelectAccountForModelWithExclusions(ctx context.Context
 // metadataUserID: 用于客户端亲和调度，从中提取客户端 ID
 // sub2apiUserID: 系统用户 ID，用于二维亲和调度
 func (s *GatewayService) SelectAccountWithLoadAwareness(ctx context.Context, groupID *int64, sessionHash string, requestedModel string, excludedIDs map[int64]struct{}, metadataUserID string, sub2apiUserID int64) (*AccountSelectionResult, error) {
-	// 渠道定价限制预检查（requested / channel_mapped 基准）
-	if s.checkChannelPricingRestriction(ctx, groupID, requestedModel) {
-		return nil, fmt.Errorf("%w supporting model: %s (channel pricing restriction)", ErrNoAvailableAccounts, requestedModel)
-	}
-
 	// 调试日志：记录调度入口参数
 	excludedIDsList := make([]int64, 0, len(excludedIDs))
 	for id := range excludedIDs {
@@ -1242,6 +1238,12 @@ func (s *GatewayService) SelectAccountWithLoadAwareness(ctx context.Context, gro
 	}
 	ctx = s.withGroupContext(ctx, group)
 
+	// Claude Code 限制可能已将 groupID 解析为 fallback group，
+	// 渠道限制预检查必须使用解析后的分组。
+	if s.checkChannelPricingRestriction(ctx, groupID, requestedModel) {
+		return nil, fmt.Errorf("%w supporting model: %s (channel pricing restriction)", ErrNoAvailableAccounts, requestedModel)
+	}
+
 	var stickyAccountID int64
 	if prefetch := prefetchedStickyAccountIDFromContext(ctx, groupID); prefetch > 0 {
 		stickyAccountID = prefetch