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/savedrecipe/entity.go

@@ -0,0 +1,76 @@
+package savedrecipe
+
+import "time"
+
+// UserSavedRecipe is a user's bookmark referencing a catalog recipe.
+// Display fields are populated by joining dishes + recipes.
+type UserSavedRecipe struct {
+	ID       string    `json:"id"`
+	UserID   string    `json:"-"`
+	RecipeID string    `json:"recipe_id"`
+	SavedAt  time.Time `json:"saved_at"`
+
+	// Display data — joined from dishes + recipes.
+	DishName    string   `json:"title"`        // dish name used as display title
+	Description *string  `json:"description"`
+	ImageURL    *string  `json:"image_url"`
+	CuisineSlug *string  `json:"cuisine_slug"`
+	Tags        []string `json:"tags"`
+
+	Difficulty  *string  `json:"difficulty"`
+	PrepTimeMin *int     `json:"prep_time_min"`
+	CookTimeMin *int     `json:"cook_time_min"`
+	Servings    *int     `json:"servings"`
+
+	CaloriesPerServing *float64 `json:"calories_per_serving"`
+	ProteinPerServing  *float64 `json:"protein_per_serving"`
+	FatPerServing      *float64 `json:"fat_per_serving"`
+	CarbsPerServing    *float64 `json:"carbs_per_serving"`
+
+	Ingredients []RecipeIngredient `json:"ingredients"`
+	Steps       []RecipeStep       `json:"steps"`
+}
+
+// RecipeIngredient is a single ingredient row.
+type RecipeIngredient struct {
+	Name       string  `json:"name"`
+	Amount     float64 `json:"amount"`
+	UnitCode   *string `json:"unit_code"`
+	IsOptional bool    `json:"is_optional"`
+}
+
+// RecipeStep is a single step row.
+type RecipeStep struct {
+	StepNumber   int    `json:"number"`
+	Description  string `json:"description"`
+	TimerSeconds *int   `json:"timer_seconds"`
+}
+
+// SaveRequest is the body for POST /saved-recipes.
+// When recipe_id is provided, the existing catalog recipe is bookmarked.
+// Otherwise a new dish+recipe is created from the supplied fields.
+type SaveRequest struct {
+	RecipeID    string `json:"recipe_id"`     // optional: bookmark existing recipe
+	Title       string `json:"title"`
+	Description string `json:"description"`
+	Cuisine     string `json:"cuisine"`
+	Difficulty  string `json:"difficulty"`
+	PrepTimeMin int    `json:"prep_time_min"`
+	CookTimeMin int    `json:"cook_time_min"`
+	Servings    int    `json:"servings"`
+	ImageURL    string `json:"image_url"`
+	// Ingredients / Steps / Tags / Nutrition are JSONB for backward compatibility
+	// with the recommendation flow that sends the full Gemini response.
+	Ingredients interface{} `json:"ingredients"`
+	Steps       interface{} `json:"steps"`
+	Tags        interface{} `json:"tags"`
+	Nutrition   interface{} `json:"nutrition_per_serving"`
+	Source      string      `json:"source"`
+}
+
+// ErrNotFound is returned when a saved recipe does not exist for the given user.
+var ErrNotFound = errorString("saved recipe not found")
+
+type errorString string
+
+func (e errorString) Error() string { return string(e) }