Add Docker Compose and backend Dockerfile

Include .gitignore, go.mod/go.sum, initial Go source, init.sql, and
Docker‑Compose
configuration for the MSSQL and backend services.

backend/go.mod

@@ -0,0 +1,11 @@
+module backend
+
+go 1.22
+
+require github.com/denisenkom/go-mssqldb v0.12.3
+
+require (
+	github.com/golang-sql/civil v0.0.0-20190719163853-cb61b32ac6fe // indirect
+	github.com/golang-sql/sqlexp v0.1.0 // indirect
+	golang.org/x/crypto v0.0.0-20220622213112-05595931fe9d // indirect
+)