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>

migrations/versions/20251206_add_word_translations.py

@@ -0,0 +1,37 @@
+"""Add word_translations table for multiple translations with context
+
+Revision ID: 20251206_word_translations
+Revises: 20251205_wordsource_ai_task
+Create Date: 2025-12-06
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '20251206_word_translations'
+down_revision: Union[str, None] = '20251205_wordsource_ai_task'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        'word_translations',
+        sa.Column('id', sa.Integer(), nullable=False),
+        sa.Column('vocabulary_id', sa.Integer(), nullable=False),
+        sa.Column('translation', sa.String(255), nullable=False),
+        sa.Column('context', sa.String(500), nullable=True),
+        sa.Column('context_translation', sa.String(500), nullable=True),
+        sa.Column('is_primary', sa.Boolean(), nullable=False, server_default='false'),
+        sa.Column('created_at', sa.DateTime(), nullable=False, server_default=sa.func.now()),
+        sa.PrimaryKeyConstraint('id')
+    )
+    op.create_index('ix_word_translations_vocabulary_id', 'word_translations', ['vocabulary_id'])
+
+
+def downgrade() -> None:
+    op.drop_index('ix_word_translations_vocabulary_id', table_name='word_translations')
+    op.drop_table('word_translations')