refactor(log): replace stdlib log with the application JSON logger

Every log.Printf/Fatalf becomes a structured slog call through
internal/log: request handlers use the request-scoped logger
(applog.FromContext, carrying request_id), startup failures exit via a
fatal() helper that still emits JSON, and the seedsample/dumpschema CLIs
bootstrap the same JSON logger. Stale client_test expectations aligned
with the refactored wrapper-call logging (message casing, result attr,
ERROR level).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/cmd/seedsample/main.go

@@ -9,22 +9,25 @@ import (
 	"context"
 	"flag"
 	"fmt"
-	"log"
+	"log/slog"
+	"os"
 	"time"
 
 	"geniusrun/backend/internal/classify"
 	"geniusrun/backend/internal/garmin"
+	applog "geniusrun/backend/internal/log"
 	"geniusrun/backend/internal/store"
 )
 
 func main() {
+	slog.SetDefault(applog.NewLogger("info", os.Stdout))
 	dbPath := flag.String("db", "geniusrun_sample.db", "path to the SQLite database to seed")
 	flag.Parse()
 
 	ctx := context.Background()
 	db, err := store.Open(*dbPath)
 	if err != nil {
-		log.Fatalf("open db: %v", err)
+		fatal("open db", err)
 	}
 	defer db.Close()
 
@@ -233,10 +236,17 @@ func seedIntervalLapsAndSamples(ctx context.Context, db *store.DB, userID, activ
 
 func must(err error) {
 	if err != nil {
-		log.Fatal(err)
+		fatal("seedsample", err)
 	}
 }
 
+// fatal logs through the application JSON logger and exits -- the
+// structured replacement for log.Fatal.
+func fatal(msg string, err error) {
+	slog.Error(msg, "error", err)
+	os.Exit(1)
+}
+
 func mustFindKindID(ctx context.Context, db *store.DB, userID int64, name string) int64 {
 	kinds, err := db.ListWorkoutKinds(ctx, userID, false)
 	must(err)
@@ -245,6 +255,7 @@ func mustFindKindID(ctx context.Context, db *store.DB, userID int64, name string
 			return k.ID
 		}
 	}
-	log.Fatalf("seedsample: no workout kind named %q found for the seeded user (did ProvisionUser seed the taxonomy?)", name)
+	slog.Error("no workout kind with that name for the seeded user (did ProvisionUser seed the taxonomy?)", "kind", name)
+	os.Exit(1)
 	return 0
 }