feat(api): add internal alerts reporting endpoint with deduplication

Add ReportAlerts endpoint for Data Plane to report alerts to Control Plane
with fingerprint-based deduplication using a 5-minute cooldown period.

Changes:
- Add POST /internal/alerts/report endpoint with validation
- Add Fingerprint field to Alert model for deduplication
- Extend GetAPIKeyStatsSummary with optional time range filtering
  using since/until query parameters to query from log records

cmd/server/main.go

@@ -300,6 +300,7 @@ func main() {
 	{
 		internalGroup.POST("/stats/flush", internalHandler.FlushStats)
 		internalGroup.POST("/apikey-stats/flush", internalHandler.FlushAPIKeyStats)
+		internalGroup.POST("/alerts/report", internalHandler.ReportAlerts)
 		internalGroup.GET("/metrics", gin.WrapH(expvar.Handler()))
 	}