refactor: replace scope-level rate limiting with model-level rate limiting

Merge functional changes from develop branch:
- Remove AntigravityQuotaScope system (claude/gemini_text/gemini_image)
- Replace with per-model rate limiting using resolveAntigravityModelKey
- Remove model load statistics (IncrModelCallCount/GetModelLoadBatch)
- Simplify account selection to unified priority→load→LRU algorithm
- Remove SetAntigravityQuotaScopeLimit from AccountRepository
- Clean up scope-related UI indicators and API fields

backend/internal/repository/gateway_cache.go

@@ -11,14 +11,6 @@ import (
 
 const stickySessionPrefix = "sticky_session:"
 
-// 模型负载统计相关常量
-const (
-	modelLoadKeyPrefix     = "ag:model_load:"      // 模型调用次数 key 前缀
-	modelLastUsedKeyPrefix = "ag:model_last_used:" // 模型最后调度时间 key 前缀
-	modelLoadTTL           = 24 * time.Hour        // 调用次数 TTL（24 小时无调用后清零）
-	modelLastUsedTTL       = 24 * time.Hour        // 最后调度时间 TTL
-)
-
 type gatewayCache struct {
 	rdb *redis.Client
 }
@@ -59,92 +51,3 @@ func (c *gatewayCache) DeleteSessionAccountID(ctx context.Context, groupID int64
 	key := buildSessionKey(groupID, sessionHash)
 	return c.rdb.Del(ctx, key).Err()
 }
-
-// ============ Antigravity 模型负载统计方法 ============
-
-// modelLoadKey 构建模型调用次数 key
-// 格式: ag:model_load:{accountID}:{model}
-func modelLoadKey(accountID int64, model string) string {
-	return fmt.Sprintf("%s%d:%s", modelLoadKeyPrefix, accountID, model)
-}
-
-// modelLastUsedKey 构建模型最后调度时间 key
-// 格式: ag:model_last_used:{accountID}:{model}
-func modelLastUsedKey(accountID int64, model string) string {
-	return fmt.Sprintf("%s%d:%s", modelLastUsedKeyPrefix, accountID, model)
-}
-
-// IncrModelCallCount 增加模型调用次数并更新最后调度时间
-// 返回更新后的调用次数
-func (c *gatewayCache) IncrModelCallCount(ctx context.Context, accountID int64, model string) (int64, error) {
-	loadKey := modelLoadKey(accountID, model)
-	lastUsedKey := modelLastUsedKey(accountID, model)
-
-	pipe := c.rdb.Pipeline()
-	incrCmd := pipe.Incr(ctx, loadKey)
-	pipe.Expire(ctx, loadKey, modelLoadTTL) // 每次调用刷新 TTL
-	pipe.Set(ctx, lastUsedKey, time.Now().Unix(), modelLastUsedTTL)
-	if _, err := pipe.Exec(ctx); err != nil {
-		return 0, err
-	}
-	return incrCmd.Val(), nil
-}
-
-// GetModelLoadBatch 批量获取账号的模型负载信息
-func (c *gatewayCache) GetModelLoadBatch(ctx context.Context, accountIDs []int64, model string) (map[int64]*service.ModelLoadInfo, error) {
-	if len(accountIDs) == 0 {
-		return make(map[int64]*service.ModelLoadInfo), nil
-	}
-
-	loadCmds, lastUsedCmds := c.pipelineModelLoadGet(ctx, accountIDs, model)
-	return c.parseModelLoadResults(accountIDs, loadCmds, lastUsedCmds), nil
-}
-
-// pipelineModelLoadGet 批量获取模型负载的 Pipeline 操作
-func (c *gatewayCache) pipelineModelLoadGet(
-	ctx context.Context,
-	accountIDs []int64,
-	model string,
-) (map[int64]*redis.StringCmd, map[int64]*redis.StringCmd) {
-	pipe := c.rdb.Pipeline()
-	loadCmds := make(map[int64]*redis.StringCmd, len(accountIDs))
-	lastUsedCmds := make(map[int64]*redis.StringCmd, len(accountIDs))
-
-	for _, id := range accountIDs {
-		loadCmds[id] = pipe.Get(ctx, modelLoadKey(id, model))
-		lastUsedCmds[id] = pipe.Get(ctx, modelLastUsedKey(id, model))
-	}
-	_, _ = pipe.Exec(ctx) // 忽略错误，key 不存在是正常的
-	return loadCmds, lastUsedCmds
-}
-
-// parseModelLoadResults 解析 Pipeline 结果
-func (c *gatewayCache) parseModelLoadResults(
-	accountIDs []int64,
-	loadCmds map[int64]*redis.StringCmd,
-	lastUsedCmds map[int64]*redis.StringCmd,
-) map[int64]*service.ModelLoadInfo {
-	result := make(map[int64]*service.ModelLoadInfo, len(accountIDs))
-	for _, id := range accountIDs {
-		result[id] = &service.ModelLoadInfo{
-			CallCount:  getInt64OrZero(loadCmds[id]),
-			LastUsedAt: getTimeOrZero(lastUsedCmds[id]),
-		}
-	}
-	return result
-}
-
-// getInt64OrZero 从 StringCmd 获取 int64 值，失败返回 0
-func getInt64OrZero(cmd *redis.StringCmd) int64 {
-	val, _ := cmd.Int64()
-	return val
-}
-
-// getTimeOrZero 从 StringCmd 获取 time.Time，失败返回零值
-func getTimeOrZero(cmd *redis.StringCmd) time.Time {
-	val, err := cmd.Int64()
-	if err != nil {
-		return time.Time{}
-	}
-	return time.Unix(val, 0)
-}