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/internal/locale/locale.go

@@ -0,0 +1,69 @@
+package locale
+
+import (
+	"context"
+	"net/http"
+	"strings"
+)
+
+// Default is the fallback language when no supported language is detected.
+const Default = "en"
+
+// Supported is the set of language codes the application currently handles.
+// Keys are ISO 639-1 two-letter codes (lower-case).
+var Supported = map[string]bool{
+	"en": true,
+	"ru": true,
+	"es": true,
+	"de": true,
+	"fr": true,
+	"it": true,
+	"pt": true,
+	"zh": true,
+	"ja": true,
+	"ko": true,
+	"ar": true,
+	"hi": true,
+}
+
+type contextKey struct{}
+
+// Parse returns the best-matching supported language from an Accept-Language
+// header value.  It iterates through the comma-separated list in preference
+// order and returns the first entry whose primary subtag is in Supported.
+// Returns Default when the header is empty or no match is found.
+func Parse(acceptLang string) string {
+	if acceptLang == "" {
+		return Default
+	}
+	for part := range strings.SplitSeq(acceptLang, ",") {
+		// Strip quality value (e.g. ";q=0.9").
+		tag := strings.TrimSpace(strings.SplitN(part, ";", 2)[0])
+		// Use only the primary subtag (e.g. "ru" from "ru-RU").
+		lang := strings.ToLower(strings.SplitN(tag, "-", 2)[0])
+		if Supported[lang] {
+			return lang
+		}
+	}
+	return Default
+}
+
+// WithLang returns a copy of ctx carrying the given language code.
+func WithLang(ctx context.Context, lang string) context.Context {
+	return context.WithValue(ctx, contextKey{}, lang)
+}
+
+// FromContext returns the language stored in ctx.
+// Returns Default when no language has been set.
+func FromContext(ctx context.Context) string {
+	if lang, ok := ctx.Value(contextKey{}).(string); ok && lang != "" {
+		return lang
+	}
+	return Default
+}
+
+// FromRequest extracts the preferred language from the request's
+// Accept-Language header.
+func FromRequest(r *http.Request) string {
+	return Parse(r.Header.Get("Accept-Language"))
+}