refactor: migrate to DDD architecture with Dishka DI

Complete architectural refactoring from simple MVC to Clean Architecture/DDD pattern:

Domain Layer:

- Add entities (Post, BaseEntity) with business logic

- Add value objects (Title, Content, Slug) with validation

- Add repository interfaces (PostRepository)

- Add domain exceptions

Application Layer:

- Add use cases (CreatePost, GetPost, UpdatePost, DeletePost, ListPosts, PublishPost)

- Add DTOs for data transfer

- Add TransactionManager interface

Infrastructure Layer:

- Add SQLAlchemy models and async database connection

- Add SQLAlchemyPostRepository implementation

- Add Dishka DI container with providers

- Add error handlers and middleware

Presentation Layer:

- Add FastAPI routes with Dishka integration

- Add Pydantic schemas

- Add dependency injection using FromDishka[T]

Other Changes:

- Remove old flat structure (api/, common/, core/, modules/)

- Add hatchling build system for package scripts

- Add blog CLI command

- Update AGENTS.md with new architecture docs

- All 48 tests passing, mypy clean, ruff clean

app/domain/value_objects/base.py

@@ -0,0 +1,37 @@
+"""Base value object for DDD domain layer."""
+
+from abc import ABC, abstractmethod
+from dataclasses import dataclass
+from typing import Any, Generic, TypeVar
+
+T = TypeVar("T")
+
+
+@dataclass(frozen=True, slots=True)
+class ValueObject(ABC, Generic[T]):
+    """Base class for all value objects."""
+
+    value: T
+
+    def __post_init__(self) -> None:
+        self._validate()
+
+    @abstractmethod
+    def _validate(self) -> None:
+        """Validate the value object. Raise ValueError if invalid."""
+        ...
+
+    def __eq__(self, other: object) -> bool:
+        if not isinstance(other, ValueObject):
+            return False
+        return bool(self.value == other.value)
+
+    def __hash__(self) -> int:
+        return hash(self.value)
+
+    def __str__(self) -> str:
+        return str(self.value)
+
+    def to_primitive(self) -> Any:
+        """Convert value object to primitive type."""
+        return self.value