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/postgres_open.go

@@ -0,0 +1,32 @@
+package infra
+
+import (
+	"database/sql"
+	"fmt"
+
+	_ "github.com/jackc/pgx/v5/stdlib"
+)
+
+// PostgresOpen connects to a PostgreSQL server and returns a *sql.DB.
+// sslmode defaults to "disable" when empty.
+// Constructs a DSN of the form:
+//
+//	host=<host> port=<port> user=<user> password=<password> dbname=<dbname> sslmode=<sslmode>
+func PostgresOpen(host string, port int, user, password, dbname string, sslmode string) (*sql.DB, error) {
+	if sslmode == "" {
+		sslmode = "disable"
+	}
+	dsn := fmt.Sprintf(
+		"host=%s port=%d user=%s password=%s dbname=%s sslmode=%s",
+		host, port, user, password, dbname, sslmode,
+	)
+	db, err := sql.Open("pgx", dsn)
+	if err != nil {
+		return nil, fmt.Errorf("postgres_open: open: %w", err)
+	}
+	if err := db.Ping(); err != nil {
+		db.Close()
+		return nil, fmt.Errorf("postgres_open: ping %s:%d/%s: %w", host, port, dbname, err)
+	}
+	return db, nil
+}