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/product/entity.go

@@ -0,0 +1,41 @@
+package product
+
+import "time"
+
+// Product is a user's food item in their pantry.
+type Product struct {
+	ID                 string    `json:"id"`
+	UserID             string    `json:"user_id"`
+	PrimaryIngredientID *string  `json:"primary_ingredient_id"`
+	Name               string    `json:"name"`
+	Quantity           float64   `json:"quantity"`
+	Unit               string    `json:"unit"`
+	Category           *string   `json:"category"`
+	StorageDays        int       `json:"storage_days"`
+	AddedAt            time.Time `json:"added_at"`
+	ExpiresAt          time.Time `json:"expires_at"`
+	DaysLeft           int       `json:"days_left"`
+	ExpiringSoon       bool      `json:"expiring_soon"`
+}
+
+// CreateRequest is the body for POST /products.
+type CreateRequest struct {
+	PrimaryIngredientID *string  `json:"primary_ingredient_id"`
+	// Accept both "primary_ingredient_id" (new) and "mapping_id" (legacy client) fields.
+	MappingID           *string  `json:"mapping_id"`
+	Name                string   `json:"name"`
+	Quantity            float64  `json:"quantity"`
+	Unit                string   `json:"unit"`
+	Category            *string  `json:"category"`
+	StorageDays         int      `json:"storage_days"`
+}
+
+// UpdateRequest is the body for PUT /products/{id}.
+// All fields are optional (nil = keep existing value).
+type UpdateRequest struct {
+	Name        *string  `json:"name"`
+	Quantity    *float64 `json:"quantity"`
+	Unit        *string  `json:"unit"`
+	Category    *string  `json:"category"`
+	StorageDays *int     `json:"storage_days"`
+}