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/internal/api/setup.go

@@ -2,7 +2,6 @@ package api
 
 import (
 	"encoding/json"
-	"log"
 	"net/http"
 	"os"
 	"path/filepath"
@@ -11,6 +10,7 @@ import (
 
 	"geniusrun/backend/internal/auth"
 	"geniusrun/backend/internal/garmin"
+	applog "geniusrun/backend/internal/log"
 )
 
 // setupSessionIdleTimeout bounds how long an onboarding Garmin session
@@ -196,7 +196,7 @@ func (s *Server) handleSetupComplete(w http.ResponseWriter, r *http.Request) {
 		oldDir := setupTokenStoreDir(s.ClientConfig.TokenStorePath, claims.Sub)
 		newDir := filepath.Join(s.ClientConfig.TokenStorePath, strconv.FormatInt(userID, 10))
 		if err := os.Rename(oldDir, newDir); err != nil && !os.IsNotExist(err) {
-			log.Printf("api: rename setup token store dir for user %d: %v", userID, err)
+			applog.FromContext(r.Context()).Error("rename setup token store dir", "user_id", userID, "error", err)
 		}
 	}