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/migrations/013_create_languages.sql

@@ -0,0 +1,24 @@
+-- +goose Up
+CREATE TABLE languages (
+    code         VARCHAR(10) PRIMARY KEY,
+    native_name  TEXT        NOT NULL,
+    english_name TEXT        NOT NULL,
+    is_active    BOOLEAN     NOT NULL DEFAULT true,
+    sort_order   SMALLINT    NOT NULL DEFAULT 0
+);
+INSERT INTO languages (code, native_name, english_name, sort_order) VALUES
+    ('en', 'English',   'English',              1),
+    ('ru', 'Русский',   'Russian',              2),
+    ('es', 'Español',   'Spanish',              3),
+    ('de', 'Deutsch',   'German',               4),
+    ('fr', 'Français',  'French',               5),
+    ('it', 'Italiano',  'Italian',              6),
+    ('pt', 'Português', 'Portuguese',           7),
+    ('zh', '中文',      'Chinese (Simplified)', 8),
+    ('ja', '日本語',    'Japanese',             9),
+    ('ko', '한국어',    'Korean',              10),
+    ('ar', 'العربية',  'Arabic',              11),
+    ('hi', 'हिन्दी',  'Hindi',               12);
+
+-- +goose Down
+DROP TABLE IF EXISTS languages;