feat: move supported languages to DB table, expose via GET /languages

- migration 013: create languages table (code PK, native_name, english_name,
  is_active, sort_order) with all 12 existing languages seeded
- locale: add Language struct, Languages []Language, LoadFromDB() — queries
  languages table at startup and populates both Supported map and Languages
  slice; existing Parse/FromContext/FromRequest unchanged
- main.go: call locale.LoadFromDB after pool is ready
- gemini/recipe.go: remove hardcoded langNames map, use locale.Languages
  linear lookup for English name in prompt
- language/handler.go: new package with GET /languages handler returning
  active languages list (no auth required)
- server.go: register GET /languages as public route
- Flutter: add LanguageRepository + languageRepositoryProvider that fetches
  /languages from backend
- language_provider.dart: replace const supportedLanguages map with
  supportedLanguagesProvider (FutureProvider) backed by LanguageRepository
- profile_provider.dart: remove supportedLanguages.containsKey validation —
  backend is source of truth; sync any non-empty language from preferences
- profile_screen.dart: use supportedLanguagesProvider for display name and
  dropdown (async with loading/error states)

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

backend/internal/language/handler.go

@@ -0,0 +1,28 @@
+package language
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/food-ai/backend/internal/locale"
+)
+
+type languageItem struct {
+	Code       string `json:"code"`
+	NativeName string `json:"native_name"`
+	SortOrder  int    `json:"sort_order"`
+}
+
+// List handles GET /languages — returns the active language list loaded from DB.
+func List(w http.ResponseWriter, r *http.Request) {
+	items := make([]languageItem, 0, len(locale.Languages))
+	for i, l := range locale.Languages {
+		items = append(items, languageItem{
+			Code:       l.Code,
+			NativeName: l.NativeName,
+			SortOrder:  i + 1,
+		})
+	}
+	w.Header().Set("Content-Type", "application/json")
+	_ = json.NewEncoder(w).Encode(map[string]any{"languages": items})
+}