feat: add generic Value[T] base class for value objects

- Create Value[T] (frozen dataclass + ABC + Generic) in domain/framework/
- Migrate ClassifiedAdTitle and ClassifiedAdText to inherit Value[str]
- Standardize all value access via .value instead of type-specific fields

tests/test_classified_ad_text.py

@@ -2,6 +2,6 @@ from domain.marketplace.classified_ad_text import ClassifiedAdText
 
 
 def test_classified_ad_text_creation():
-    text = ClassifiedAdText(text="This is a classified ad description.")
+    text = ClassifiedAdText(value="This is a classified ad description.")
 
-    assert text.text == "This is a classified ad description."
+    assert text.value == "This is a classified ad description."