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/menu.go

@@ -4,34 +4,14 @@ import (
 	"context"
 	"fmt"
 	"sync"
+
+	"github.com/food-ai/backend/internal/adapters/ai"
 )
 
-// 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"`
-}
-
 // GenerateMenu produces a 7-day × 3-meal plan by issuing three parallel
 // GenerateRecipes calls (one per meal type). This avoids token-limit errors
 // that arise from requesting 21 full recipes in a single prompt.
-func (c *Client) GenerateMenu(ctx context.Context, req MenuRequest) ([]DayPlan, error) {
+func (c *Client) GenerateMenu(ctx context.Context, req ai.MenuRequest) ([]ai.DayPlan, error) {
 	type mealSlot struct {
 		mealType string
 		fraction float64 // share of daily calories
@@ -44,7 +24,7 @@ func (c *Client) GenerateMenu(ctx context.Context, req MenuRequest) ([]DayPlan,
 	}
 
 	type mealResult struct {
-		recipes []Recipe
+		recipes []ai.Recipe
 		err     error
 	}
 
@@ -57,7 +37,7 @@ func (c *Client) GenerateMenu(ctx context.Context, req MenuRequest) ([]DayPlan,
 			defer wg.Done()
 			// Scale daily calories to what this meal should contribute.
 			mealCal := int(float64(req.DailyCalories) * fraction)
-			r, err := c.GenerateRecipes(ctx, RecipeRequest{
+			r, err := c.GenerateRecipes(ctx, ai.RecipeRequest{
 				UserGoal:          req.UserGoal,
 				DailyCalories:     mealCal * 3, // prompt divides by 3 internally
 				Restrictions:      req.Restrictions,
@@ -84,11 +64,11 @@ func (c *Client) GenerateMenu(ctx context.Context, req MenuRequest) ([]DayPlan,
 		}
 	}
 
-	days := make([]DayPlan, 7)
+	days := make([]ai.DayPlan, 7)
 	for day := range 7 {
-		days[day] = DayPlan{
+		days[day] = ai.DayPlan{
 			Day: day + 1,
-			Meals: []MealEntry{
+			Meals: []ai.MealEntry{
 				{MealType: slots[0].mealType, Recipe: results[0].recipes[day]},
 				{MealType: slots[1].mealType, Recipe: results[1].recipes[day]},
 				{MealType: slots[2].mealType, Recipe: results[2].recipes[day]},