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

domain/marketplace/classified_ad_text.py

@@ -1,6 +1,8 @@
 from dataclasses import dataclass
 
+from domain.framework.value import Value
+
 
 @dataclass(frozen=True)
-class ClassifiedAdText:
-    text: str
+class ClassifiedAdText(Value[str]):
+    pass