fix: fix menu generation errors and show planned meals on home screen

Backend fixes:
- migration 003: add 'menu' value to recipe_source enum (was causing SQLSTATE 22P02)
- migration 004: rename recipe_products→recipe_ingredients, product_id→ingredient_id (was causing SQLSTATE 42P01)
- dish/repository.go: fix INSERT INTO tags using $1/$1 for two columns → $1/$2 (was causing SQLSTATE 42P08)
- home/handler.go: replace non-existent saved_recipes table with correct joins (recipes→dishes→dish_translations, user_saved_recipes) so today's plan and recommendations load correctly
- reqlog: new slog.Handler wrapper that adds request_id and stack trace to ERROR-level logs
- all handlers: slog.Error→slog.ErrorContext so error logs include request context; writeError includes request_id in response body

Client:
- home_screen.dart: extend home screen to future dates, show planned meals as ghost entries
- l10n: add new localisation keys for home screen date navigation and planned meal UI

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

backend/internal/domain/dish/repository.go

@@ -312,13 +312,20 @@ func (r *Repository) Create(ctx context.Context, req CreateRequest) (recipeID st
 		return "", fmt.Errorf("insert dish: %w", err)
 	}
 
-	// Insert tags.
+	// Insert tags — upsert into tags first so the FK constraint is satisfied
+	// even when the AI generates a tag slug that does not exist yet.
 	for _, slug := range req.Tags {
-		if _, err := tx.Exec(ctx,
+		if _, upsertErr := tx.Exec(ctx,
+			`INSERT INTO tags (slug, name) VALUES ($1, $2) ON CONFLICT (slug) DO NOTHING`,
+			slug, slug,
+		); upsertErr != nil {
+			return "", fmt.Errorf("upsert tag %s: %w", slug, upsertErr)
+		}
+		if _, insertErr := tx.Exec(ctx,
 			`INSERT INTO dish_tags (dish_id, tag_slug) VALUES ($1, $2) ON CONFLICT DO NOTHING`,
 			dishID, slug,
-		); err != nil {
-			return "", fmt.Errorf("insert dish tag %s: %w", slug, err)
+		); insertErr != nil {
+			return "", fmt.Errorf("insert dish tag %s: %w", slug, insertErr)
 		}
 	}