init(db): initialize db module

also add go.mod and go.sum

internal/db/db.go

@@ -0,0 +1,29 @@
+package db
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+var connPool *pgxpool.Pool
+
+func InitDB(connString string) error {
+	poolConfig, err := pgxpool.ParseConfig(connString)
+	if err != nil {
+		return fmt.Errorf("error while parsing connection string: %w", err)
+	}
+
+	poolConfig.MaxConns = 10
+	poolConfig.MinConns = 0
+	poolConfig.MaxConnLifetime = time.Hour
+	poolConfig.HealthCheckPeriod = 30 * time.Second
+
+	connPool, err = pgxpool.NewWithConfig(context.Background(), poolConfig)
+	if err != nil {
+		return fmt.Errorf("error while initializing DB connection pool: %w", err)
+	}
+	return nil
+}