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/schemas.py

@@ -1,12 +1,30 @@
+from typing import Literal
+
 from pydantic import BaseModel, EmailStr, Field
 
+SUPPORTED_LANGS = ("en", "it", "es", "fr", "de")
+
 
 class WaitlistRequest(BaseModel):
     email: EmailStr
+    lang: Literal["en", "it", "es", "fr", "de"] = "en"
     # Honeypot field — must be empty. Bots tend to fill hidden fields.
     website: str = Field(default="", max_length=0)
 
 
+_RESPONSE_MESSAGES: dict[str, str] = {
+    "en": "Check your inbox for a confirmation link!",
+    "it": "Controlla la tua casella email per il link di conferma!",
+    "es": "¡Revisa tu bandeja de entrada para el enlace de confirmación!",
+    "fr": "Vérifiez votre boîte de réception pour le lien de confirmation !",
+    "de": "Überprüfe deinen Posteingang für den Bestätigungslink!",
+}
+
+
 class WaitlistResponse(BaseModel):
     ok: bool = True
     message: str = "Check your inbox for a confirmation link!"
+
+    @classmethod
+    def for_lang(cls, lang: str = "en") -> "WaitlistResponse":
+        return cls(message=_RESPONSE_MESSAGES.get(lang, _RESPONSE_MESSAGES["en"]))