Convert backend from submodule to regular directory

Remove submodule tracking; backend is now a plain directory in the repo.
Also update deploy workflow: remove --recurse-submodules.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/cmd/createuser/main.go

@@ -0,0 +1,55 @@
+// cmd/createuser creates a new user in the database.
+// Usage: DATABASE_URL=... go run ./cmd/createuser <username> <password>
+package main
+
+import (
+	"context"
+	"fmt"
+	"os"
+
+	"github.com/jackc/pgx/v5"
+
+	"github.com/jacek/pamietnik/backend/internal/auth"
+)
+
+func main() {
+	if len(os.Args) != 3 {
+		fmt.Fprintln(os.Stderr, "usage: createuser <username> <password>")
+		os.Exit(1)
+	}
+	username := os.Args[1]
+	password := os.Args[2]
+
+	if len(password) < 8 {
+		fmt.Fprintln(os.Stderr, "password must be at least 8 characters")
+		os.Exit(1)
+	}
+
+	dsn := os.Getenv("DATABASE_URL")
+	if dsn == "" {
+		dsn = "postgres://pamietnik:pamietnik@localhost:5432/pamietnik?sslmode=disable"
+	}
+
+	conn, err := pgx.Connect(context.Background(), dsn)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "db error:", err)
+		os.Exit(1)
+	}
+	defer conn.Close(context.Background())
+
+	hash, err := auth.HashPassword(password)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "hash error:", err)
+		os.Exit(1)
+	}
+
+	_, err = conn.Exec(context.Background(),
+		`INSERT INTO users (username, password_hash) VALUES ($1, $2)`,
+		username, hash,
+	)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "insert error:", err)
+		os.Exit(1)
+	}
+	fmt.Printf("user '%s' created\n", username)
+}