refactor: enhance config handling and unify main entrypoint

Unify configuration parsing logic across entrypoints and clean up legacy script-side config paths.

internal/app/etl/loaders/process.go

@@ -0,0 +1,43 @@
+package loaders
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"git.ksdemosapps.com/kylesoda/go-migrate/internal/app/config"
+	"git.ksdemosapps.com/kylesoda/go-migrate/internal/app/custom_errors"
+	"git.ksdemosapps.com/kylesoda/go-migrate/internal/app/models"
+	"github.com/jackc/pgx/v5/pgconn"
+)
+
+func (gl *GenericLoader) ProcessBatch(
+	ctx context.Context,
+	tableInfo config.TargetTableInfo,
+	colNames []string,
+	batch models.Batch,
+) (int64, error) {
+	_, err := gl.db.SaveMassive(
+		ctx,
+		tableInfo.Schema,
+		tableInfo.Table,
+		colNames,
+		batch.Rows,
+	)
+
+	if err != nil {
+		if pgErr, ok := errors.AsType[*pgconn.PgError](err); ok {
+			if pgErr.Code == "23505" {
+				return 0, &custom_errors.JobError{
+					ShouldCancelJob: true,
+					Msg:             fmt.Sprintf("Fatal error in table %s.%s", tableInfo.Schema, tableInfo.Table),
+					Prev:            err,
+				}
+			}
+		}
+
+		return 0, &custom_errors.LoaderError{Batch: batch, Msg: err.Error()}
+	}
+
+	return int64(len(batch.Rows)), nil
+}