ax(database): add usage-example comment to GetHashrateStats

GetHashrateStats had no comment at all — bare exported function.
AX Principle 2 requires comments as usage examples, not prose.

Co-Authored-By: Charon <charon@lethean.io>

pkg/database/hashrate.go

@@ -125,6 +125,9 @@ type HashrateStats struct {
 	LastSeen    time.Time `json:"lastSeen"`
 }
 
+// GetHashrateStats retrieves aggregated statistics for a specific miner.
+// stats, err := database.GetHashrateStats("xmrig")
+// if stats != nil { log.Printf("avg: %d", stats.AverageRate) }
 func GetHashrateStats(minerName string) (*HashrateStats, error) {
 	databaseMutex.RLock()
 	defer databaseMutex.RUnlock()