refactor: introduce reqctx pkg for request context

apps/backend/internal/reqctx/reqctx.go

@@ -0,0 +1,23 @@
+package reqctx
+
+import (
+	"errors"
+
+	"github.com/gofiber/fiber/v2"
+)
+
+const authenticatedUserKey = "authenticatedUser"
+
+var ErrUnauthenticatedRequest = errors.New("unauthenticated request")
+
+// AuthenticatedUser returns the authenticated user from the given fiber context.
+// Returns ErrUnauthenticatedRequest if not authenticated.
+// The caller must type assert the returned value to the appropriate user type.
+func AuthenticatedUser(c *fiber.Ctx) any {
+	return c.Locals(authenticatedUserKey)
+}
+
+// SetAuthenticatedUser sets the authenticated user in the fiber context.
+func SetAuthenticatedUser(c *fiber.Ctx, user interface{}) {
+	c.Locals(authenticatedUserKey, user)
+}