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/framework/__init__.py

@@ -0,0 +1 @@
+from .value import Value as Value

domain/framework/value.py

@@ -0,0 +1,7 @@
+from abc import ABC
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True)
+class Value[T](ABC):
+    value: T