feat: multiple translations with context, improved task examples

- Add WordTranslation model for storing multiple translations per word
- AI generates translations with example sentences and their translations
- Show example usage after answering tasks (learning + interface language)
- Save translations to word_translations table when adding words from tasks
- Improve word exclusion in new_words mode (stronger prompt + client filtering)
- Add migration for word_translations table

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

locales/ja.json

@@ -58,6 +58,7 @@
     "prompt": "追加したい単語を送ってください:\n例: <code>/add elephant</code>\n\nコマンドなしで単語だけ送ってもOKです！",
     "searching": "⏳ 翻訳と例を検索中...",
     "examples_header": "<b>例文:</b>",
+    "translations_header": "<b>翻訳:</b>",
     "translation_label": "翻訳",
     "category_label": "カテゴリー",
     "level_label": "レベル",
@@ -124,6 +125,7 @@
     "add_word_btn": "➕ 単語を追加",
     "word_added": "✅ 単語 '{word}' を単語帳に追加しました！",
     "word_already_exists": "単語 '{word}' はすでに単語帳にあります",
+    "example_label": "例文",
     "cancelled": "キャンセルしました。/task で課題に戻れます。",
     "finish_title": "{emoji} <b>課題が終了しました！</b>",
     "correct_of": "正解数: <b>{correct}</b> / {total}",