feat: implement backend localization infrastructure

- Add internal/locale package: Parse(Accept-Language), FromContext/WithLang helpers, 12 supported languages
- Add Language middleware that reads Accept-Language header and stores lang in context
- Register Language middleware globally in server router (after CORS)

Database migrations:
- 009: create recipe_translations, saved_recipe_translations, ingredient_translations tables; migrate existing _ru data
- 010: drop legacy _ru columns (title_ru, description_ru, canonical_name_ru); update FTS index

Models: remove all _ru fields (TitleRu, DescriptionRu, NameRu, UnitRu, CanonicalNameRu)

Repositories:
- recipe: Upsert drops _ru params; GetByID does LEFT JOIN COALESCE on recipe_translations; ListMissingTranslation(lang); UpsertTranslation
- ingredient: same pattern with ingredient_translations; Search now queries translated names/aliases
- savedrecipe: List/GetByID LEFT JOIN COALESCE on saved_recipe_translations; UpsertTranslation

Gemini:
- RecipeRequest/MenuRequest gain Lang field
- buildRecipePrompt rewritten in English with target-language content instruction; image_query always in English
- GenerateMenu propagates Lang to GenerateRecipes

Handlers:
- recommendation/menu: pass locale.FromContext(ctx) as Lang
- recognition: saveClassification stores Russian translation via UpsertTranslation instead of _ru column

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

backend/migrations/010_drop_ru_columns.sql

@@ -0,0 +1,36 @@
+-- +goose Up
+
+-- Drop the full-text search index that references the soon-to-be-removed
+-- title_ru column.
+DROP INDEX IF EXISTS idx_recipes_title_fts;
+
+-- Remove legacy _ru columns from recipes now that the data lives in
+-- recipe_translations (migration 009).
+ALTER TABLE recipes
+    DROP COLUMN IF EXISTS title_ru,
+    DROP COLUMN IF EXISTS description_ru;
+
+-- Remove the legacy Russian name column from ingredient_mappings.
+ALTER TABLE ingredient_mappings
+    DROP COLUMN IF EXISTS canonical_name_ru;
+
+-- Recreate the FTS index on the English title only.
+-- Cross-language search is now handled at the application level by querying
+-- the appropriate translation row.
+CREATE INDEX idx_recipes_title_fts ON recipes
+    USING GIN (to_tsvector('simple', coalesce(title, '')));
+
+-- +goose Down
+DROP INDEX IF EXISTS idx_recipes_title_fts;
+
+ALTER TABLE recipes
+    ADD COLUMN title_ru       VARCHAR(500),
+    ADD COLUMN description_ru TEXT;
+
+ALTER TABLE ingredient_mappings
+    ADD COLUMN canonical_name_ru VARCHAR(255);
+
+-- Restore the bilingual FTS index.
+CREATE INDEX idx_recipes_title_fts ON recipes
+    USING GIN (to_tsvector('simple',
+        coalesce(title_ru, '') || ' ' || coalesce(title, '')));