feat: 用量统计页改版，月度汇总支持多指标图表

- 新增 GET /api/usage/monthly?year= 年度按自然月聚合，每月含按模型分解（token 降序）
- 月度汇总改为堆叠柱状图：Token/消费金额/调用次数三指标切换，按模型分色（图例取前 8，其余归入「其他」）
- 选中月份概览卡片（金额/次数/token 分解），点击柱体或图例联动切换
- 年份切换、悬停明细 tooltip、请求明细保留

backend/internal/api/usage.go

@@ -1,7 +1,9 @@
 package api
 
 import (
+	"fmt"
 	"net/http"
+	"sort"
 	"strconv"
 	"time"
 
@@ -77,6 +79,130 @@ func (h *Handler) MyUsageStats(c *gin.Context) {
 	})
 }
 
+// MyUsageMonthly GET /api/usage/monthly?year=2026 — 当前用户年度按自然月聚合，
+// 每月含按模型分解（供月度堆叠柱状图使用）。
+func (h *Handler) MyUsageMonthly(c *gin.Context) {
+	userID, _ := c.Get("user_id")
+	uid, _ := userID.(uint64)
+
+	year := time.Now().Year()
+	if y := c.Query("year"); y != "" {
+		if n, err := strconv.Atoi(y); err == nil && n >= 2000 && n <= 2100 {
+			year = n
+		}
+	}
+	start := time.Date(year, 1, 1, 0, 0, 0, 0, time.Local)
+	end := start.AddDate(1, 0, -1)
+
+	dailies, err := h.dailyDAO.ListByDateRange(c.Request.Context(), uid, start, end)
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "failed to load usage"})
+		return
+	}
+
+	// 补齐模型名（模型可能已被删除，回退为 模型#id）
+	modelIDs := make([]uint64, 0, len(dailies))
+	seen := map[uint64]bool{}
+	for _, d := range dailies {
+		if !seen[d.ModelID] {
+			seen[d.ModelID] = true
+			modelIDs = append(modelIDs, d.ModelID)
+		}
+	}
+	modelNames := map[uint64]string{}
+	if len(modelIDs) > 0 {
+		var models []store.Model
+		if err := h.db.Where("id IN ?", modelIDs).Find(&models).Error; err == nil {
+			for _, m := range models {
+				modelNames[m.ID] = m.Name
+			}
+		}
+	}
+
+	type modelAgg struct {
+		ModelID         uint64  `json:"model_id"`
+		ModelName       string  `json:"model_name"`
+		Requests        int64   `json:"requests"`
+		InputTokens     int64   `json:"input_tokens"`
+		OutputTokens    int64   `json:"output_tokens"`
+		CacheReadTokens int64   `json:"cache_read_tokens"`
+		Cost            float64 `json:"cost"`
+	}
+	type monthAgg struct {
+		Month           string               `json:"month"`
+		Requests        int64                `json:"requests"`
+		InputTokens     int64                `json:"input_tokens"`
+		OutputTokens    int64                `json:"output_tokens"`
+		CacheReadTokens int64                `json:"cache_read_tokens"`
+		Cost            float64              `json:"cost"`
+		Models          map[uint64]*modelAgg `json:"-"`
+	}
+
+	months := make([]*monthAgg, 12)
+	for i := range months {
+		months[i] = &monthAgg{
+			Month:  fmt.Sprintf("%d-%02d", year, i+1),
+			Models: map[uint64]*modelAgg{},
+		}
+	}
+	for _, d := range dailies {
+		mm, err := strconv.Atoi(d.Date[5:7])
+		if err != nil || mm < 1 || mm > 12 {
+			continue
+		}
+		m := months[mm-1]
+		m.Requests += d.Requests
+		m.InputTokens += d.InputTokens
+		m.OutputTokens += d.OutputTokens
+		m.CacheReadTokens += d.CacheReadTokens
+		m.Cost += d.Cost
+		ma, ok := m.Models[d.ModelID]
+		if !ok {
+			name := modelNames[d.ModelID]
+			if name == "" {
+				name = fmt.Sprintf("模型#%d", d.ModelID)
+			}
+			ma = &modelAgg{ModelID: d.ModelID, ModelName: name}
+			m.Models[d.ModelID] = ma
+		}
+		ma.Requests += d.Requests
+		ma.InputTokens += d.InputTokens
+		ma.OutputTokens += d.OutputTokens
+		ma.CacheReadTokens += d.CacheReadTokens
+		ma.Cost += d.Cost
+	}
+
+	out := make([]gin.H, 12)
+	for i, m := range months {
+		modelList := make([]*modelAgg, 0, len(m.Models))
+		for _, ma := range m.Models {
+			modelList = append(modelList, ma)
+		}
+		// 模型按 token 总量降序，柱状图图例顺序与之一致
+		sort.Slice(modelList, func(a, b int) bool {
+			ta := modelList[a].InputTokens + modelList[a].OutputTokens + modelList[a].CacheReadTokens
+			tb := modelList[b].InputTokens + modelList[b].OutputTokens + modelList[b].CacheReadTokens
+			return ta > tb
+		})
+		out[i] = gin.H{
+			"month":              m.Month,
+			"requests":           m.Requests,
+			"input_tokens":       m.InputTokens,
+			"output_tokens":      m.OutputTokens,
+			"cache_read_tokens":  m.CacheReadTokens,
+			"cost":               m.Cost,
+			"models":             modelList,
+		}
+	}
+
+	c.JSON(http.StatusOK, gin.H{
+		"data": gin.H{
+			"year":   year,
+			"months": out,
+		},
+	})
+}
+
 // MyUsageLogs GET /api/usage/logs?page=1&pageSize=20 — 当前用户的用量明细（分页）。
 func (h *Handler) MyUsageLogs(c *gin.Context) {
 	userID, _ := c.Get("user_id")