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/infra/middleware/auth.go

@@ -0,0 +1,56 @@
+package middleware
+
+import (
+	"context"
+	"net/http"
+	"strings"
+)
+
+const (
+	userIDKey   contextKey = "user_id"
+	userPlanKey contextKey = "user_plan"
+)
+
+// TokenClaims represents the result of validating an access token.
+type TokenClaims struct {
+	UserID string
+	Plan   string
+}
+
+// AccessTokenValidator validates JWT access tokens.
+type AccessTokenValidator interface {
+	ValidateAccessToken(tokenStr string) (*TokenClaims, error)
+}
+
+func Auth(validator AccessTokenValidator) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			header := r.Header.Get("Authorization")
+			if !strings.HasPrefix(header, "Bearer ") {
+				http.Error(w, `{"error":"unauthorized"}`, http.StatusUnauthorized)
+				return
+			}
+
+			tokenStr := strings.TrimPrefix(header, "Bearer ")
+			claims, err := validator.ValidateAccessToken(tokenStr)
+			if err != nil {
+				http.Error(w, `{"error":"invalid token"}`, http.StatusUnauthorized)
+				return
+			}
+
+			ctx := context.WithValue(r.Context(), userIDKey, claims.UserID)
+			ctx = context.WithValue(ctx, userPlanKey, claims.Plan)
+			next.ServeHTTP(w, r.WithContext(ctx))
+		})
+	}
+}
+
+func UserIDFromCtx(ctx context.Context) string {
+	id, _ := ctx.Value(userIDKey).(string)
+	return id
+}
+
+func UserPlanFromCtx(ctx context.Context) string {
+	plan, _ := ctx.Value(userPlanKey).(string)
+	return plan
+}