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/repository/internal500_counter_cache.go

@@ -0,0 +1,55 @@
+package repository
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/Wei-Shaw/sub2api/internal/service"
+	"github.com/redis/go-redis/v9"
+)
+
+const (
+	internal500CounterPrefix    = "internal500_count:account:"
+	internal500CounterTTLSeconds = 86400 // 24 小时兜底
+)
+
+// internal500CounterIncrScript 使用 Lua 脚本原子性地增加计数并返回当前值
+// 如果 key 不存在，则创建并设置过期时间
+var internal500CounterIncrScript = redis.NewScript(`
+	local key = KEYS[1]
+	local ttl = tonumber(ARGV[1])
+
+	local count = redis.call('INCR', key)
+	if count == 1 then
+		redis.call('EXPIRE', key, ttl)
+	end
+
+	return count
+`)
+
+type internal500CounterCache struct {
+	rdb *redis.Client
+}
+
+// NewInternal500CounterCache 创建 INTERNAL 500 连续失败计数器缓存实例
+func NewInternal500CounterCache(rdb *redis.Client) service.Internal500CounterCache {
+	return &internal500CounterCache{rdb: rdb}
+}
+
+// IncrementInternal500Count 原子递增计数并返回当前值
+func (c *internal500CounterCache) IncrementInternal500Count(ctx context.Context, accountID int64) (int64, error) {
+	key := fmt.Sprintf("%s%d", internal500CounterPrefix, accountID)
+
+	result, err := internal500CounterIncrScript.Run(ctx, c.rdb, []string{key}, internal500CounterTTLSeconds).Int64()
+	if err != nil {
+		return 0, fmt.Errorf("increment internal500 count: %w", err)
+	}
+
+	return result, nil
+}
+
+// ResetInternal500Count 清零计数器（成功响应时调用）
+func (c *internal500CounterCache) ResetInternal500Count(ctx context.Context, accountID int64) error {
+	key := fmt.Sprintf("%s%d", internal500CounterPrefix, accountID)
+	return c.rdb.Del(ctx, key).Err()
+}