refactor: introduce adapter pattern for AI provider (OpenAI)

- Add internal/adapters/ai/types.go with neutral shared types
  (Recipe, DayPlan, RecognizedItem, IngredientClassification, etc.)
- Remove types from internal/adapters/openai/ — adapter now uses ai.*
- Define Recognizer interface in recognition package
- Define MenuGenerator interface in menu package
- Define RecipeGenerator interface in recommendation package
- Handler structs now hold interfaces, not *openai.Client
- Add wire.Bind entries for the three new interface bindings

To swap OpenAI for another provider: implement the three interfaces
using ai.* types and change the wire.Bind lines in cmd/server/wire.go.

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

backend/internal/adapters/ai/types.go

@@ -0,0 +1,128 @@
+package ai
+
+// RecipeRequest contains parameters for recipe generation.
+type RecipeRequest struct {
+	UserGoal          string   // "lose" | "maintain" | "gain"
+	DailyCalories     int
+	Restrictions      []string // e.g. ["gluten_free", "vegetarian"]
+	CuisinePrefs      []string // e.g. ["russian", "asian"]
+	Count             int
+	AvailableProducts []string // human-readable list of products in user's pantry
+	Lang              string   // ISO 639-1 target language code, e.g. "en", "ru"
+}
+
+// Recipe is an AI-generated recipe.
+type Recipe struct {
+	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"`
+	ImageQuery  string        `json:"image_query"`
+	ImageURL    string        `json:"image_url"`
+	Ingredients []Ingredient  `json:"ingredients"`
+	Steps       []Step        `json:"steps"`
+	Tags        []string      `json:"tags"`
+	Nutrition   NutritionInfo `json:"nutrition_per_serving"`
+}
+
+// Ingredient is a single ingredient in a recipe.
+type Ingredient struct {
+	Name   string  `json:"name"`
+	Amount float64 `json:"amount"`
+	Unit   string  `json:"unit"`
+}
+
+// Step is a single preparation step.
+type Step struct {
+	Number       int    `json:"number"`
+	Description  string `json:"description"`
+	TimerSeconds *int   `json:"timer_seconds"`
+}
+
+// NutritionInfo contains approximate nutritional information per serving.
+type NutritionInfo struct {
+	Calories    float64 `json:"calories"`
+	ProteinG    float64 `json:"protein_g"`
+	FatG        float64 `json:"fat_g"`
+	CarbsG      float64 `json:"carbs_g"`
+	Approximate bool    `json:"approximate"`
+}
+
+// MenuRequest contains parameters for weekly menu generation.
+type MenuRequest struct {
+	UserGoal          string
+	DailyCalories     int
+	Restrictions      []string
+	CuisinePrefs      []string
+	AvailableProducts []string
+	Lang              string // ISO 639-1 target language code, e.g. "en", "ru"
+}
+
+// DayPlan is the AI-generated plan for a single day.
+type DayPlan struct {
+	Day   int         `json:"day"`
+	Meals []MealEntry `json:"meals"`
+}
+
+// MealEntry is a single meal within a day plan.
+type MealEntry struct {
+	MealType string `json:"meal_type"` // breakfast | lunch | dinner
+	Recipe   Recipe `json:"recipe"`
+}
+
+// RecognizedItem is a food item identified in an image.
+type RecognizedItem struct {
+	Name       string  `json:"name"`
+	Quantity   float64 `json:"quantity"`
+	Unit       string  `json:"unit"`
+	Category   string  `json:"category"`
+	Confidence float64 `json:"confidence"`
+}
+
+// UnrecognizedItem is text from a receipt that could not be identified as food.
+type UnrecognizedItem struct {
+	RawText string  `json:"raw_text"`
+	Price   float64 `json:"price,omitempty"`
+}
+
+// ReceiptResult is the full result of receipt OCR.
+type ReceiptResult struct {
+	Items        []RecognizedItem   `json:"items"`
+	Unrecognized []UnrecognizedItem `json:"unrecognized"`
+}
+
+// DishResult is the result of dish recognition.
+type DishResult struct {
+	DishName      string   `json:"dish_name"`
+	WeightGrams   int      `json:"weight_grams"`
+	Calories      float64  `json:"calories"`
+	ProteinG      float64  `json:"protein_g"`
+	FatG          float64  `json:"fat_g"`
+	CarbsG        float64  `json:"carbs_g"`
+	Confidence    float64  `json:"confidence"`
+	SimilarDishes []string `json:"similar_dishes"`
+}
+
+// IngredientTranslation holds the localized name and aliases for one language.
+type IngredientTranslation struct {
+	Lang    string   `json:"lang"`
+	Name    string   `json:"name"`
+	Aliases []string `json:"aliases"`
+}
+
+// IngredientClassification is the AI-produced classification of an unknown food item.
+type IngredientClassification struct {
+	CanonicalName   string                  `json:"canonical_name"`
+	Aliases         []string                `json:"aliases"`      // English aliases
+	Translations    []IngredientTranslation `json:"translations"` // other languages
+	Category        string                  `json:"category"`
+	DefaultUnit     string                  `json:"default_unit"`
+	CaloriesPer100g *float64                `json:"calories_per_100g"`
+	ProteinPer100g  *float64                `json:"protein_per_100g"`
+	FatPer100g      *float64                `json:"fat_per_100g"`
+	CarbsPer100g    *float64                `json:"carbs_per_100g"`
+	StorageDays     int                     `json:"storage_days"`
+}