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>

database/models.py

@@ -93,6 +93,19 @@ class Vocabulary(Base):
     notes: Mapped[Optional[str]] = mapped_column(String(500))  # Заметки пользователя
 
 
+class WordTranslation(Base):
+    """Модель перевода слова с контекстом"""
+    __tablename__ = "word_translations"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    vocabulary_id: Mapped[int] = mapped_column(Integer, nullable=False, index=True)
+    translation: Mapped[str] = mapped_column(String(255), nullable=False)
+    context: Mapped[Optional[str]] = mapped_column(String(500))  # Пример предложения
+    context_translation: Mapped[Optional[str]] = mapped_column(String(500))  # Перевод примера
+    is_primary: Mapped[bool] = mapped_column(Boolean, default=False)  # Основной перевод
+    created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow)
+
+
 class Task(Base):
     """Модель задания"""
     __tablename__ = "tasks"