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/migrations/014_create_units.sql

@@ -0,0 +1,58 @@
+-- +goose Up
+
+CREATE TABLE units (
+    code       VARCHAR(20) PRIMARY KEY,
+    sort_order SMALLINT    NOT NULL DEFAULT 0
+);
+INSERT INTO units (code, sort_order) VALUES
+    ('g',    1),
+    ('kg',   2),
+    ('ml',   3),
+    ('l',    4),
+    ('pcs',  5),
+    ('pack', 6);
+
+CREATE TABLE unit_translations (
+    unit_code  VARCHAR(20) NOT NULL REFERENCES units(code) ON DELETE CASCADE,
+    lang       VARCHAR(10) NOT NULL,
+    name       TEXT        NOT NULL,
+    PRIMARY KEY (unit_code, lang)
+);
+INSERT INTO unit_translations (unit_code, lang, name) VALUES
+    ('g',    'ru', 'г'),
+    ('kg',   'ru', 'кг'),
+    ('ml',   'ru', 'мл'),
+    ('l',    'ru', 'л'),
+    ('pcs',  'ru', 'шт'),
+    ('pack', 'ru', 'уп');
+
+-- Normalize products.unit from Russian display strings to English codes
+UPDATE products SET unit = 'g'    WHERE unit = 'г';
+UPDATE products SET unit = 'kg'   WHERE unit = 'кг';
+UPDATE products SET unit = 'ml'   WHERE unit = 'мл';
+UPDATE products SET unit = 'l'    WHERE unit = 'л';
+UPDATE products SET unit = 'pcs'  WHERE unit = 'шт';
+UPDATE products SET unit = 'pack' WHERE unit = 'уп';
+-- Normalize any remaining unknown values
+UPDATE products SET unit = 'pcs' WHERE unit NOT IN (SELECT code FROM units);
+
+-- Nullify unknown default_unit values in ingredient_mappings before adding FK
+UPDATE ingredient_mappings
+    SET default_unit = NULL
+    WHERE default_unit IS NOT NULL
+      AND default_unit NOT IN (SELECT code FROM units);
+
+-- Foreign key constraints
+ALTER TABLE products
+    ADD CONSTRAINT fk_product_unit
+    FOREIGN KEY (unit) REFERENCES units(code);
+
+ALTER TABLE ingredient_mappings
+    ADD CONSTRAINT fk_default_unit
+    FOREIGN KEY (default_unit) REFERENCES units(code);
+
+-- +goose Down
+ALTER TABLE ingredient_mappings DROP CONSTRAINT IF EXISTS fk_default_unit;
+ALTER TABLE products             DROP CONSTRAINT IF EXISTS fk_product_unit;
+DROP TABLE IF EXISTS unit_translations;
+DROP TABLE IF EXISTS units;