feat(api): add alert system with CRUD endpoints and statistics

Introduce a comprehensive alert management system for monitoring
system events and notifications.

Changes include:
- Add Alert model with type, severity, status, and metadata fields
- Implement AlertHandler with full CRUD operations (create, list,
  get, acknowledge, resolve, dismiss)
- Add alert statistics endpoint for counts by status and severity
- Register Alert model in database auto-migration
- Add minute-level aggregation to log stats (limited to 6-hour range)

internal/api/log_handler.go

@@ -235,6 +235,8 @@ type GroupedStatsItem struct {
 	Month string `json:"month,omitempty"`
 	// For group_by=hour
 	Hour string `json:"hour,omitempty"`
+	// For group_by=minute
+	Minute string `json:"minute,omitempty"`
 
 	Count        int64   `json:"count"`
 	TokensIn     int64   `json:"tokens_in"`
@@ -348,24 +350,29 @@ func (h *Handler) deleteLogsBefore(cutoff time.Time, keyID uint, modelName strin
 
 // LogStats godoc
 // @Summary      Log stats (admin)
-// @Description  Aggregate log stats with basic filtering. Use group_by param for grouped statistics (model/day/month/hour). Without group_by returns LogStatsResponse; with group_by returns GroupedStatsResponse.
+// @Description  Aggregate log stats with basic filtering. Use group_by param for grouped statistics (model/day/month/hour/minute). Without group_by returns LogStatsResponse; with group_by returns GroupedStatsResponse. Note: minute-level aggregation is limited to 6-hour time ranges.
 // @Tags         admin
 // @Produce      json
 // @Security     AdminAuth
 // @Param        since    query     int     false  "unix seconds"
 // @Param        until    query     int     false  "unix seconds"
-// @Param        group_by query     string  false  "group by dimension: model, day, month, hour. Returns GroupedStatsResponse when specified."  Enums(model, day, month, hour)
+// @Param        group_by query     string  false  "group by dimension: model, day, month, hour, minute. Returns GroupedStatsResponse when specified."  Enums(model, day, month, hour, minute)
 // @Success      200  {object}  LogStatsResponse  "Default aggregated stats (when group_by is not specified)"
 // @Success      200  {object}  GroupedStatsResponse  "Grouped stats (when group_by is specified)"
+// @Failure      400  {object}  gin.H
 // @Failure      500  {object}  gin.H
 // @Router       /admin/logs/stats [get]
 func (h *Handler) LogStats(c *gin.Context) {
 	q := h.logBaseQuery()
+
+	var sinceTime, untilTime *time.Time
 	if t, ok := parseUnixSeconds(c.Query("since")); ok {
 		q = q.Where("created_at >= ?", t)
+		sinceTime = &t
 	}
 	if t, ok := parseUnixSeconds(c.Query("until")); ok {
 		q = q.Where("created_at <= ?", t)
+		untilTime = &t
 	}
 
 	groupBy := strings.TrimSpace(c.Query("group_by"))
@@ -382,6 +389,9 @@ func (h *Handler) LogStats(c *gin.Context) {
 	case "hour":
 		h.logStatsByHour(c, q)
 		return
+	case "minute":
+		h.logStatsByMinute(c, q, sinceTime, untilTime)
+		return
 	}
 
 	// Default: aggregated stats (backward compatible)
@@ -544,6 +554,61 @@ func (h *Handler) logStatsByHour(c *gin.Context, q *gorm.DB) {
 	c.JSON(http.StatusOK, GroupedStatsResponse{Items: items})
 }
 
+// maxMinuteRangeDuration is the maximum time range allowed for minute-level aggregation (6 hours)
+const maxMinuteRangeDuration = 6 * time.Hour
+
+// logStatsByMinute handles group_by=minute with time range validation
+func (h *Handler) logStatsByMinute(c *gin.Context, q *gorm.DB, sinceTime, untilTime *time.Time) {
+	// Validate time range - minute-level aggregation requires since/until and max 6 hours
+	if sinceTime == nil || untilTime == nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "minute-level aggregation requires both 'since' and 'until' parameters"})
+		return
+	}
+
+	duration := untilTime.Sub(*sinceTime)
+	if duration > maxMinuteRangeDuration {
+		c.JSON(http.StatusBadRequest, gin.H{
+			"error":       "time range too large for minute-level aggregation",
+			"max_hours":   6,
+			"actual_hours": duration.Hours(),
+		})
+		return
+	}
+
+	if duration < 0 {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "'until' must be after 'since'"})
+		return
+	}
+
+	type minuteStats struct {
+		Minute       string
+		Cnt          int64
+		TokensIn     int64
+		TokensOut    int64
+		AvgLatencyMs float64
+	}
+	var rows []minuteStats
+	// PostgreSQL DATE_TRUNC for minute-level aggregation
+	if err := q.Select(`DATE_TRUNC('minute', created_at) as minute, COUNT(*) as cnt, COALESCE(SUM(tokens_in),0) as tokens_in, COALESCE(SUM(tokens_out),0) as tokens_out, COALESCE(AVG(latency_ms),0) as avg_latency_ms`).
+		Group("minute").
+		Order("minute ASC").
+		Scan(&rows).Error; err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "failed to aggregate by minute", "details": err.Error()})
+		return
+	}
+	items := make([]GroupedStatsItem, 0, len(rows))
+	for _, r := range rows {
+		items = append(items, GroupedStatsItem{
+			Minute:       r.Minute,
+			Count:        r.Cnt,
+			TokensIn:     r.TokensIn,
+			TokensOut:    r.TokensOut,
+			AvgLatencyMs: r.AvgLatencyMs,
+		})
+	}
+	c.JSON(http.StatusOK, GroupedStatsResponse{Items: items})
+}
+
 // ListSelfLogs godoc
 // @Summary      List logs (master)
 // @Description  List request logs for the authenticated master