feat: dynamic units table with localized names via GET /units

- Add units + unit_translations tables with FK constraints on products and ingredient_mappings
- Normalize products.unit from Russian strings (г, кг) to English codes (g, kg)
- Load units at startup (in-memory registry) and serve via GET /units (language-aware)
- Replace hardcoded _units lists and _mapUnit() functions in Flutter with unitsProvider FutureProvider
- Re-fetches automatically when language changes

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

backend/cmd/server/main.go

@@ -20,6 +20,7 @@ import (
 	"github.com/food-ai/backend/internal/locale"
 	"github.com/food-ai/backend/internal/menu"
 	"github.com/food-ai/backend/internal/middleware"
+	"github.com/food-ai/backend/internal/units"
 	"github.com/food-ai/backend/internal/pexels"
 	"github.com/food-ai/backend/internal/product"
 	"github.com/food-ai/backend/internal/recognition"
@@ -78,6 +79,11 @@ func run() error {
 	}
 	slog.Info("languages loaded", "count", len(locale.Languages))
 
+	if err := units.LoadFromDB(ctx, pool); err != nil {
+		return fmt.Errorf("load units: %w", err)
+	}
+	slog.Info("units loaded", "count", len(units.Records))
+
 	// Firebase auth
 	firebaseAuth, err := auth.NewFirebaseAuthOrNoop(cfg.FirebaseCredentialsFile)
 	if err != nil {