feat: add translation language setting & onboarding flow

- Add separate translation_language setting (independent from interface language)
- Implement 3-step onboarding for new users:
  1. Choose interface language
  2. Choose learning language
  3. Choose translation language
- Fix localization issues when using callback.message (user_id from state)
- Add UserService.get_user_by_id() method
- Add get_user_translation_lang() helper in i18n
- Update all handlers to use correct translation language
- Add localization keys for onboarding (ru/en/ja)

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

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

migrations/versions/20251207_add_translation_language.py

@@ -0,0 +1,25 @@
+"""Add translation_language field to users table
+
+Revision ID: 20251207_translation_language
+Revises: 20251206_word_translations
+Create Date: 2025-12-07
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '20251207_translation_language'
+down_revision: Union[str, None] = '20251206_word_translations'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column('users', sa.Column('translation_language', sa.String(2), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column('users', 'translation_language')