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/openai/recipe.go

@@ -6,65 +6,10 @@ import (
 	"fmt"
 	"strings"
 
+	"github.com/food-ai/backend/internal/adapters/ai"
 	"github.com/food-ai/backend/internal/infra/locale"
 )
 
-// RecipeGenerator generates recipes using the Gemini AI.
-type RecipeGenerator interface {
-	GenerateRecipes(ctx context.Context, req RecipeRequest) ([]Recipe, error)
-}
-
-// 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 a recipe returned by Gemini.
-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"`
-}
-
 // goalNames maps internal goal codes to English descriptions used in the prompt.
 var goalNames = map[string]string{
 	"lose":     "weight loss",
@@ -72,10 +17,10 @@ var goalNames = map[string]string{
 	"gain":     "muscle gain",
 }
 
-// GenerateRecipes generates recipes using the Gemini AI.
+// GenerateRecipes generates recipes using the OpenAI API.
 // Retries up to maxRetries times only when the response is not valid JSON.
 // API-level errors (rate limits, auth, etc.) are returned immediately.
-func (c *Client) GenerateRecipes(ctx context.Context, req RecipeRequest) ([]Recipe, error) {
+func (c *Client) GenerateRecipes(ctx context.Context, req ai.RecipeRequest) ([]ai.Recipe, error) {
 	prompt := buildRecipePrompt(req)
 
 	messages := []map[string]string{
@@ -111,7 +56,7 @@ func (c *Client) GenerateRecipes(ctx context.Context, req RecipeRequest) ([]Reci
 	return nil, fmt.Errorf("failed to parse valid JSON after %d attempts: %w", maxRetries, lastErr)
 }
 
-func buildRecipePrompt(req RecipeRequest) string {
+func buildRecipePrompt(req ai.RecipeRequest) string {
 	lang := req.Lang
 	if lang == "" {
 		lang = "en"
@@ -189,7 +134,7 @@ Return ONLY a valid JSON array without markdown or extra text:
 }]`, count, langName, goal, req.DailyCalories, restrictions, cuisines, productsSection, perMealCalories, langName)
 }
 
-func parseRecipesJSON(text string) ([]Recipe, error) {
+func parseRecipesJSON(text string) ([]ai.Recipe, error) {
 	text = strings.TrimSpace(text)
 	if strings.HasPrefix(text, "```") {
 		text = strings.TrimPrefix(text, "```json")
@@ -198,7 +143,7 @@ func parseRecipesJSON(text string) ([]Recipe, error) {
 		text = strings.TrimSpace(text)
 	}
 
-	var recipes []Recipe
+	var recipes []ai.Recipe
 	if err := json.Unmarshal([]byte(text), &recipes); err != nil {
 		return nil, err
 	}