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

alembic/versions/003_add_language_column.py

@@ -0,0 +1,23 @@
+"""add language column to waitlist_entries
+
+Revision ID: 003
+Revises: 002
+Create Date: 2026-04-12
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "003"
+down_revision: Union[str, None] = "002"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("waitlist_entries", sa.Column("language", sa.String(5), nullable=False, server_default="en"))
+
+
+def downgrade() -> None:
+    op.drop_column("waitlist_entries", "language")