Use random string in ref link instead of tg ID

- Enable old links with tg id by default in .env with LEGACY_REFS=true

db/models.py

@@ -21,6 +21,7 @@ class User(Base):
                                server_default=func.now())
     is_banned = Column(Boolean, default=False)
     panel_user_uuid = Column(String, nullable=True, unique=True, index=True)
+    referral_code = Column(String(16), nullable=True, unique=True, index=True)
     referred_by_id = Column(BigInteger,
                             ForeignKey("users.user_id"),
                             nullable=True)