feat(api): add structured JSON access log with request-id correlation

backend/cmd/geniusrund/main.go

@@ -6,12 +6,15 @@ package main
 import (
 	"context"
 	"log"
+	"log/slog"
 	"net/http"
+	"os"
 	"os/signal"
 	"syscall"
 	"time"
 
 	"geniusrun/backend/internal/api"
+	"geniusrun/backend/internal/applog"
 	"geniusrun/backend/internal/auth"
 	"geniusrun/backend/internal/config"
 	"geniusrun/backend/internal/garmin"
@@ -24,6 +27,7 @@ func main() {
 	if err != nil {
 		log.Fatalf("config: %v", err)
 	}
+	slog.SetDefault(applog.NewLogger(cfg.LogLevel, os.Stdout))
 
 	db, err := store.Open(cfg.DBPath)
 	if err != nil {