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/internal/database/seed/seed.go

@@ -0,0 +1,38 @@
+package seed
+
+import (
+	"context"
+	"database/sql"
+	"embed"
+	"fmt"
+)
+
+//go:embed *.sql
+var files embed.FS
+
+func Apply(ctx context.Context, db *sql.DB, environment string) error {
+	if _, err := apply(ctx, db, "common.sql"); err != nil {
+		return err
+	}
+	switch environment {
+	case "", "production":
+		return nil
+	case "development", "test":
+		_, err := apply(ctx, db, environment+".sql")
+		return err
+	default:
+		return fmt.Errorf("unsupported APP_ENV %q", environment)
+	}
+}
+
+func apply(ctx context.Context, db *sql.DB, filename string) (sql.Result, error) {
+	content, err := files.ReadFile(filename)
+	if err != nil {
+		return nil, fmt.Errorf("read seed %s: %w", filename, err)
+	}
+	result, err := db.ExecContext(ctx, string(content))
+	if err != nil {
+		return nil, fmt.Errorf("apply seed %s: %w", filename, err)
+	}
+	return result, nil
+}