fix: correct return type for SQLALCHEMY_DATABASE_URI computed field

Replace MultiHostUrl.build() with PostgresDsn.build() to match the
PostgresDsn return type annotation and resolve type checker error

Author: myrkytyn

backend/app/core/config.py

@@ -60,7 +60,7 @@ def all_cors_origins(self) -> list[str]:
     @computed_field  # type: ignore[prop-decorator]
     @property
     def SQLALCHEMY_DATABASE_URI(self) -> PostgresDsn:
-        return MultiHostUrl.build(
+        return PostgresDsn.build(
             scheme="postgresql+psycopg",
             username=self.POSTGRES_USER,
             password=self.POSTGRES_PASSWORD,