feat(i18n): add multilanguage support to waitlist emails and result pages

- Add 'language' column to waitlist_entries (en/it/es/fr/de, default en)
- Accept 'lang' field in POST /waitlist request body
- Translate confirmation email (subject, badge, heading, body, CTA, footer)
- Translate confirm/unsubscribe result HTML pages
- Return localized success message in WaitlistResponse
- Update language preference on duplicate signups
- Alembic migration 003_add_language_column

app/models.py

@@ -20,6 +20,7 @@ class WaitlistEntry(Base):
     email: Mapped[str] = mapped_column(String(320), unique=True, nullable=False, index=True)
     ip_address: Mapped[str | None] = mapped_column(String(45), nullable=True)
     source: Mapped[str | None] = mapped_column(String(64), nullable=True)
+    language: Mapped[str] = mapped_column(String(5), nullable=False, server_default=sa.text("'en'"))
     confirmed: Mapped[bool] = mapped_column(Boolean, default=False, server_default=sa.text("0"))
     consent_given_at: Mapped[datetime.datetime | None] = mapped_column(
         DateTime(timezone=True), nullable=True,