Switch backend to wait for SQL Server and use direct INSERTs

Replace stored procedure calls with inline INSERT statements and
SCOPE_IDENTITY()
Add netcat for readiness check in new entrypoint.sh
Introduce dedicated SQL Server and init-db services in docker-compose

backend/main.go

@@ -41,17 +41,9 @@ func main() {
 	// -----------------------------------------------------------------
 	var newCustID int
 	insertCustSQL := `
-DECLARE @NewID int;
-EXEC dbo.sp_InsertCustomer
-    @CustomerName = @name,
-    @AddressLine1 = @addr1,
-    @AddressLine2 = @addr2,
-    @City = @city,
-    @State = @state,
-    @ZipCode = @zip,
-    @Country = @country,
-    @NewCustomerID = @NewID OUTPUT;
-SELECT @NewID;
+INSERT INTO dbo.Customers (CustomerName, AddressLine1, AddressLine2, City, State, ZipCode, Country)
+VALUES (@name, @addr1, @addr2, @city, @state, @zip, @country);
+SELECT SCOPE_IDENTITY() AS NewID;
 `
 	row := db.QueryRow(insertCustSQL,
 		sql.Named("name", "Acme Corp"),
@@ -72,14 +64,9 @@ SELECT @NewID;
 	// -----------------------------------------------------------------
 	var newContactID int
 	insertContactSQL := `
-DECLARE @NewContactID int;
-EXEC dbo.sp_InsertContact
-    @CustomerID = @custID,
-    @ContactTypeID = @typeID,
-    @ContactValue = @value,
-    @IsPrimary = @primary,
-    @NewContactID = @NewContactID OUTPUT;
-SELECT @NewContactID;
+INSERT INTO dbo.Contacts (CustomerID, ContactTypeID, ContactValue, IsPrimary)
+VALUES (@custID, @typeID, @value, @primary);
+SELECT SCOPE_IDENTITY() AS NewContactID;
 `
 	// Assume ContactTypeID 1 = Phone (created by init.sql)
 	row = db.QueryRow(insertContactSQL,