feat: implement Iteration 1 — AI recipe recommendations

Backend:
- Add Groq LLM client (llama-3.3-70b) for recipe generation with JSON
  retry strategy (retries only on parse errors, not API errors)
- Add Pexels client for parallel photo search per recipe
- Add saved_recipes table (migration 004) with JSONB fields
- Add GET /recommendations endpoint (profile-aware prompt building)
- Add POST/GET/GET{id}/DELETE /saved-recipes CRUD endpoints
- Wire gemini, pexels, recommendation, savedrecipe packages in main.go

Flutter:
- Add Recipe, SavedRecipe models with json_serializable
- Add RecipeService (getRecommendations, getSavedRecipes, save, delete)
- Add RecommendationsNotifier and SavedRecipesNotifier (Riverpod)
- Add RecommendationsScreen with skeleton loading and refresh FAB
- Add RecipeDetailScreen (SliverAppBar, nutrition tooltip, steps with timer)
- Add SavedRecipesScreen with Dismissible swipe-to-delete and empty state
- Update RecipesScreen to TabBar (Recommendations / Saved)
- Add /recipe-detail route outside ShellRoute (no bottom nav)
- Extend ApiClient with getList() and deleteVoid()

Project:
- Add CLAUDE.md with English-only rule for comments and commit messages

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

backend/internal/recommendation/handler.go

@@ -0,0 +1,138 @@
+package recommendation
+
+import (
+	"context"
+	"encoding/json"
+	"log/slog"
+	"net/http"
+	"strconv"
+	"sync"
+
+	"github.com/food-ai/backend/internal/gemini"
+	"github.com/food-ai/backend/internal/middleware"
+	"github.com/food-ai/backend/internal/user"
+)
+
+// PhotoSearcher can search for a photo by text query.
+type PhotoSearcher interface {
+	SearchPhoto(ctx context.Context, query string) (string, error)
+}
+
+// UserLoader can load a user profile by ID.
+type UserLoader interface {
+	GetByID(ctx context.Context, id string) (*user.User, error)
+}
+
+// userPreferences is the shape of user.Preferences JSONB.
+type userPreferences struct {
+	Cuisines     []string `json:"cuisines"`
+	Restrictions []string `json:"restrictions"`
+}
+
+// Handler handles GET /recommendations.
+type Handler struct {
+	gemini     *gemini.Client
+	pexels     PhotoSearcher
+	userLoader UserLoader
+}
+
+// NewHandler creates a new Handler.
+func NewHandler(geminiClient *gemini.Client, pexels PhotoSearcher, userLoader UserLoader) *Handler {
+	return &Handler{
+		gemini:     geminiClient,
+		pexels:     pexels,
+		userLoader: userLoader,
+	}
+}
+
+// GetRecommendations handles GET /recommendations?count=5.
+func (h *Handler) GetRecommendations(w http.ResponseWriter, r *http.Request) {
+	userID := middleware.UserIDFromCtx(r.Context())
+	if userID == "" {
+		writeErrorJSON(w, http.StatusUnauthorized, "unauthorized")
+		return
+	}
+
+	count := 5
+	if s := r.URL.Query().Get("count"); s != "" {
+		if n, err := strconv.Atoi(s); err == nil && n > 0 && n <= 20 {
+			count = n
+		}
+	}
+
+	u, err := h.userLoader.GetByID(r.Context(), userID)
+	if err != nil {
+		slog.Error("load user for recommendations", "user_id", userID, "err", err)
+		writeErrorJSON(w, http.StatusInternalServerError, "failed to load user profile")
+		return
+	}
+
+	req := buildRecipeRequest(u, count)
+
+	recipes, err := h.gemini.GenerateRecipes(r.Context(), req)
+	if err != nil {
+		slog.Error("generate recipes", "user_id", userID, "err", err)
+		writeErrorJSON(w, http.StatusServiceUnavailable, "recipe generation failed, please try again")
+		return
+	}
+
+	// Fetch Pexels photos in parallel — each goroutine owns a distinct index.
+	var wg sync.WaitGroup
+	for i := range recipes {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			imageURL, err := h.pexels.SearchPhoto(r.Context(), recipes[i].ImageQuery)
+			if err != nil {
+				slog.Warn("pexels photo search failed", "query", recipes[i].ImageQuery, "err", err)
+			}
+			recipes[i].ImageURL = imageURL
+		}(i)
+	}
+	wg.Wait()
+
+	writeJSON(w, http.StatusOK, recipes)
+}
+
+func buildRecipeRequest(u *user.User, count int) gemini.RecipeRequest {
+	req := gemini.RecipeRequest{
+		Count:         count,
+		DailyCalories: 2000, // sensible default
+	}
+
+	if u.Goal != nil {
+		req.UserGoal = *u.Goal
+	}
+	if u.DailyCalories != nil && *u.DailyCalories > 0 {
+		req.DailyCalories = *u.DailyCalories
+	}
+
+	if len(u.Preferences) > 0 {
+		var prefs userPreferences
+		if err := json.Unmarshal(u.Preferences, &prefs); err == nil {
+			req.CuisinePrefs = prefs.Cuisines
+			req.Restrictions = prefs.Restrictions
+		}
+	}
+	return req
+}
+
+type errorResponse struct {
+	Error string `json:"error"`
+}
+
+func writeErrorJSON(w http.ResponseWriter, status int, msg string) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	if err := json.NewEncoder(w).Encode(errorResponse{Error: msg}); err != nil {
+		slog.Error("write error response", "err", err)
+	}
+}
+
+func writeJSON(w http.ResponseWriter, status int, v any) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	if err := json.NewEncoder(w).Encode(v); err != nil {
+		slog.Error("write JSON response", "err", err)
+	}
+}