refactor: restructure internal/ into adapters/, infra/, and app layers

- internal/gemini/ → internal/adapters/openai/ (renamed package to openai)
- internal/pexels/ → internal/adapters/pexels/
- internal/config/   → internal/infra/config/
- internal/database/ → internal/infra/database/
- internal/locale/   → internal/infra/locale/
- internal/middleware/ → internal/infra/middleware/
- internal/server/   → internal/infra/server/

All import paths and call sites updated accordingly.

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

backend/internal/adapters/openai/menu.go

@@ -0,0 +1,99 @@
+package openai
+
+import (
+	"context"
+	"fmt"
+	"sync"
+)
+
+// 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) {
+	type mealSlot struct {
+		mealType string
+		fraction float64 // share of daily calories
+	}
+
+	slots := []mealSlot{
+		{"breakfast", 0.25},
+		{"lunch", 0.40},
+		{"dinner", 0.35},
+	}
+
+	type mealResult struct {
+		recipes []Recipe
+		err     error
+	}
+
+	results := make([]mealResult, len(slots))
+	var wg sync.WaitGroup
+
+	for i, slot := range slots {
+		wg.Add(1)
+		go func(idx int, mealType string, fraction float64) {
+			defer wg.Done()
+			// Scale daily calories to what this meal should contribute.
+			mealCal := int(float64(req.DailyCalories) * fraction)
+			r, err := c.GenerateRecipes(ctx, RecipeRequest{
+				UserGoal:          req.UserGoal,
+				DailyCalories:     mealCal * 3, // prompt divides by 3 internally
+				Restrictions:      req.Restrictions,
+				CuisinePrefs:      req.CuisinePrefs,
+				Count:             7,
+				AvailableProducts: req.AvailableProducts,
+				Lang:              req.Lang,
+			})
+			results[idx] = mealResult{r, err}
+		}(i, slot.mealType, slot.fraction)
+	}
+	wg.Wait()
+
+	for i, res := range results {
+		if res.err != nil {
+			return nil, fmt.Errorf("generate %s: %w", slots[i].mealType, res.err)
+		}
+		if len(res.recipes) == 0 {
+			return nil, fmt.Errorf("no %s recipes returned", slots[i].mealType)
+		}
+		// Pad to exactly 7 by repeating the last recipe.
+		for len(results[i].recipes) < 7 {
+			results[i].recipes = append(results[i].recipes, results[i].recipes[len(results[i].recipes)-1])
+		}
+	}
+
+	days := make([]DayPlan, 7)
+	for day := range 7 {
+		days[day] = DayPlan{
+			Day: day + 1,
+			Meals: []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]},
+			},
+		}
+	}
+	return days, nil
+}