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/store/users.go

@@ -56,14 +56,14 @@ var defaultWorkoutKindSeeds = []WorkoutKind{
 // default workout kinds (with their paired workout_type_paces rows), and an
 // initial sync_state row -- all in one transaction, so a partially
 // provisioned user is never observable.
-func (db *DB) ProvisionUser(ctx context.Context, oidcSub, displayName string) (int64, error) {
+func (db *DB) ProvisionUser(ctx context.Context, oidcSub, name string) (int64, error) {
 	tx, err := db.BeginTx(ctx, nil)
 	if err != nil {
 		return 0, fmt.Errorf("begin provision user tx: %w", err)
 	}
 	defer tx.Rollback()
 
-	res, err := tx.ExecContext(ctx, `INSERT INTO users (oidc_sub, name) VALUES (?, ?)`, oidcSub, displayName)
+	res, err := tx.ExecContext(ctx, `INSERT INTO users (oidc_sub, name) VALUES (?, ?)`, oidcSub, name)
 	if err != nil {
 		return 0, fmt.Errorf("create user %q: %w", oidcSub, err)
 	}