feat: funciones impuras migration_create, migration_up, migration_down, migration_status

Fase 2 del issue 0015. MigrationCreate (crea archivo .sql template con version
auto-calculada), MigrationUp (aplica migraciones pendientes en transacciones
individuales), MigrationDown (revierte ultimas N via down_sql de _migrations),
MigrationGetStatus (cruza disco con BD, detecta orphaned). Tests de integracion:
ciclo completo create->up->status->down->status. 26 tests, todos pasan.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

functions/infra/migration_down.go

@@ -0,0 +1,79 @@
+package infra
+
+import (
+	"database/sql"
+	"fmt"
+	"time"
+)
+
+// MigrationDown reverts the last n applied migrations by executing their down_sql
+// from the _migrations table. Migrations are reverted in reverse version order
+// (highest version first). Each reversion runs in its own transaction.
+// Returns the list of reverted migrations. If n <= 0, no migrations are reverted.
+func MigrationDown(db *sql.DB, n int) ([]Migration, error) {
+	if n <= 0 {
+		return nil, nil
+	}
+
+	// Fetch last n applied migrations in descending order
+	const query = `
+		SELECT version, name, up_sql, down_sql, applied_at
+		FROM _migrations
+		ORDER BY version DESC
+		LIMIT ?`
+
+	rows, err := db.Query(query, n)
+	if err != nil {
+		return nil, fmt.Errorf("migration_down: query _migrations: %w", err)
+	}
+	defer rows.Close()
+
+	var toRevert []Migration
+	for rows.Next() {
+		var m Migration
+		var appliedAtStr string
+		if err := rows.Scan(&m.Version, &m.Name, &m.UpSQL, &m.DownSQL, &appliedAtStr); err != nil {
+			return nil, fmt.Errorf("migration_down: scan row: %w", err)
+		}
+		m.AppliedAt, _ = time.Parse("2006-01-02 15:04:05", appliedAtStr)
+		toRevert = append(toRevert, m)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, fmt.Errorf("migration_down: rows error: %w", err)
+	}
+
+	// Revert each migration in its own transaction (already in DESC order)
+	var reverted []Migration
+	for _, m := range toRevert {
+		if err := revertMigration(db, m); err != nil {
+			return reverted, fmt.Errorf("migration_down: reverting version %d (%s): %w", m.Version, m.Name, err)
+		}
+		reverted = append(reverted, m)
+	}
+
+	return reverted, nil
+}
+
+// revertMigration executes a migration's DownSQL within a transaction and removes it
+// from _migrations. If DownSQL is empty, only the record is removed.
+func revertMigration(db *sql.DB, m Migration) error {
+	tx, err := db.Begin()
+	if err != nil {
+		return fmt.Errorf("begin transaction: %w", err)
+	}
+	defer tx.Rollback() //nolint:errcheck
+
+	// Execute the down SQL if present
+	if m.DownSQL != "" {
+		if _, err := tx.Exec(m.DownSQL); err != nil {
+			return fmt.Errorf("exec down_sql: %w", err)
+		}
+	}
+
+	// Remove the migration record
+	if _, err := tx.Exec("DELETE FROM _migrations WHERE version = ?", m.Version); err != nil {
+		return fmt.Errorf("delete from _migrations: %w", err)
+	}
+
+	return tx.Commit()
+}