Add configurable welcome bonus days for referred signups

- add REFERRAL_WELCOME_BONUS_DAYS setting (default 3) in settings.py
- document REFERRAL_WELCOME_BONUS_DAYS in .env.example
- apply welcome bonus on first /start only for newly created users with referred_by_id
- replace hardcoded 3 days with settings.REFERRAL_WELCOME_BONUS_DAYS
- skip bonus flow when value is 0 or less
- send user notification after successful bonus application
- add i18n key referral_welcome_bonus_applied to ru.json and en.json

config/settings.py

@@ -170,6 +170,10 @@ class Settings(BaseSettings):
         default=True,
         description="When true, referral bonuses (for inviter and referee) are applied only once per invited user - on their first successful payment."
     )
+    REFERRAL_WELCOME_BONUS_DAYS: int = Field(
+        default=3,
+        description="Welcome bonus days granted to a newly registered user who joined via referral link.",
+    )
     LEGACY_REFS: bool = Field(
         default=True,
         description="Allow legacy referral links like ref_<telegram_id> to continue working. Defaults to True when unset."