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>

database/models.py

@@ -55,8 +55,9 @@ class User(Base):
     id: Mapped[int] = mapped_column(primary_key=True)
     telegram_id: Mapped[int] = mapped_column(BigInteger, unique=True, nullable=False)
     username: Mapped[Optional[str]] = mapped_column(String(255))
-    language_interface: Mapped[str] = mapped_column(String(2), default="ru")  # ru/en
-    learning_language: Mapped[str] = mapped_column(String(2), default="en")  # en
+    language_interface: Mapped[str] = mapped_column(String(2), default="ru")  # ru/en/ja - UI language
+    learning_language: Mapped[str] = mapped_column(String(2), default="en")  # en/ja - language being learned
+    translation_language: Mapped[Optional[str]] = mapped_column(String(2), default=None)  # ru/en/ja - translation target (defaults to language_interface if None)
     level: Mapped[LanguageLevel] = mapped_column(SQLEnum(LanguageLevel), default=LanguageLevel.A1)
     levels_by_language: Mapped[Optional[dict]] = mapped_column(JSON, default=None)  # {"en": "B1", "ja": "N4"}
     timezone: Mapped[str] = mapped_column(String(50), default="UTC")