feat: abstracción DB multi-engine — CRUD genérico y openers para SQLite, Postgres, ClickHouse, DuckDB

Funciones Go con interfaz unificada para operaciones DB: open, close, create_table, exec, query, insert_row, insert_batch.
Openers específicos por engine. Tipo DBConfig para configuración común.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

functions/infra/duckdb_open.go

@@ -0,0 +1,27 @@
+package infra
+
+import (
+	"database/sql"
+	"fmt"
+
+	_ "github.com/marcboeker/go-duckdb"
+)
+
+// DuckDBOpen opens (or creates) a DuckDB database file.
+// Pass an empty path or ":memory:" for an in-memory database.
+// Returns a ready-to-use *sql.DB or an error.
+func DuckDBOpen(path string) (*sql.DB, error) {
+	dsn := path
+	if dsn == "" {
+		dsn = ":memory:"
+	}
+	db, err := sql.Open("duckdb", dsn)
+	if err != nil {
+		return nil, fmt.Errorf("duckdb_open: open %q: %w", dsn, err)
+	}
+	if err := db.Ping(); err != nil {
+		db.Close()
+		return nil, fmt.Errorf("duckdb_open: ping %q: %w", dsn, err)
+	}
+	return db, nil
+}