feat(antigravity): progressive penalty for consecutive INTERNAL 500 errors

When an antigravity account returns 500 "Internal error encountered."
on all 3 retry attempts, increment a Redis counter and apply escalating
penalties:
- 1st round: temp unschedulable 10 minutes
- 2nd round: temp unschedulable 10 hours
- 3rd round: permanently mark as error

Counter resets on any successful response (< 400).

backend/internal/service/internal500_counter.go

@@ -0,0 +1,11 @@
+package service
+
+import "context"
+
+// Internal500CounterCache 追踪 Antigravity 账号连续 INTERNAL 500 失败轮数
+type Internal500CounterCache interface {
+	// IncrementInternal500Count 原子递增计数并返回当前值
+	IncrementInternal500Count(ctx context.Context, accountID int64) (int64, error)
+	// ResetInternal500Count 清零计数器（成功响应时调用）
+	ResetInternal500Count(ctx context.Context, accountID int64) error
+}