test: add self-contained integration environment

Separate schema migrations from environment-specific fixtures, run the full API workflow against a disposable PostgreSQL Testcontainer, and provide seeded development and UI test environments.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/cmd/seed/main.go

@@ -0,0 +1,33 @@
+package main
+
+import (
+	"context"
+	"log/slog"
+	"os"
+
+	"github.com/danyel/go-guess/backend/internal/database"
+	"github.com/danyel/go-guess/backend/internal/database/seed"
+)
+
+func main() {
+	databaseURL := os.Getenv("DATABASE_URL")
+	if databaseURL == "" {
+		slog.Error("DATABASE_URL is required")
+		os.Exit(1)
+	}
+	db, err := database.Open(databaseURL)
+	if err != nil {
+		slog.Error("connect database", "error", err)
+		os.Exit(1)
+	}
+	sqlDB, err := db.DB()
+	if err != nil {
+		slog.Error("get database connection", "error", err)
+		os.Exit(1)
+	}
+	defer sqlDB.Close()
+	if err := seed.Apply(context.Background(), sqlDB, os.Getenv("APP_ENV")); err != nil {
+		slog.Error("seed database", "error", err)
+		os.Exit(1)
+	}
+}