fix(usage): 用户使用记录不下发账号计费倍率

- 将 usage log DTO 拆分为用户/管理员两类
- 用户接口不返回 account_rate_multiplier/ip_address/account
- 管理员接口保留管理员字段
- 补充契约测试防止回归

backend/internal/server/api_contract_test.go

@@ -190,24 +190,25 @@ func TestAPIContracts(t *testing.T) {
 				t.Helper()
 				deps.usageRepo.SetUserLogs(1, []service.UsageLog{
 					{
-						ID:                  1,
-						UserID:              1,
-						APIKeyID:            100,
-						AccountID:           200,
-						RequestID:           "req_123",
-						Model:               "claude-3",
-						InputTokens:         10,
-						OutputTokens:        20,
-						CacheCreationTokens: 1,
-						CacheReadTokens:     2,
-						TotalCost:           0.5,
-						ActualCost:          0.5,
-						RateMultiplier:      1,
-						BillingType:         service.BillingTypeBalance,
-						Stream:              true,
-						DurationMs:          ptr(100),
-						FirstTokenMs:        ptr(50),
-						CreatedAt:           deps.now,
+						ID:                    1,
+						UserID:                1,
+						APIKeyID:              100,
+						AccountID:             200,
+						AccountRateMultiplier: ptr(0.5),
+						RequestID:             "req_123",
+						Model:                 "claude-3",
+						InputTokens:           10,
+						OutputTokens:          20,
+						CacheCreationTokens:   1,
+						CacheReadTokens:       2,
+						TotalCost:             0.5,
+						ActualCost:            0.5,
+						RateMultiplier:        1,
+						BillingType:           service.BillingTypeBalance,
+						Stream:                true,
+						DurationMs:            ptr(100),
+						FirstTokenMs:          ptr(50),
+						CreatedAt:             deps.now,
 					},
 				})
 			},
@@ -238,10 +239,9 @@ func TestAPIContracts(t *testing.T) {
 							"output_cost": 0,
 							"cache_creation_cost": 0,
 							"cache_read_cost": 0,
-							"total_cost": 0.5,
+						"total_cost": 0.5,
 						"actual_cost": 0.5,
 						"rate_multiplier": 1,
-						"account_rate_multiplier": null,
 						"billing_type": 0,
 							"stream": true,
 							"duration_ms": 100,