refactor: introduce internal/domain/ layer, rename model.go → entity.go

Move all business-logic packages from internal/ root into internal/domain/:
  auth, cuisine, diary, dish, home, ingredient, language, menu, product,
  recipe, recognition, recommendation, savedrecipe, tag, units, user

Rename model.go → entity.go in packages that hold domain entities:
  diary, dish, home, ingredient, menu, product, recipe, savedrecipe, user

Update all import paths accordingly (adapters, infra/server, cmd/server,
tests). No logic changes.

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

backend/internal/domain/auth/token_verifier.go

@@ -0,0 +1,8 @@
+package auth
+
+import "context"
+
+// TokenVerifier abstracts token verification for testability.
+type TokenVerifier interface {
+	VerifyToken(ctx context.Context, idToken string) (uid, email, name, avatarURL string, err error)
+}