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/entities/base.py

@@ -0,0 +1,33 @@
+"""Base entity for DDD domain layer."""
+
+from abc import ABC, abstractmethod
+from dataclasses import dataclass, field
+from datetime import datetime, timezone
+from typing import Any
+from uuid import UUID, uuid4
+
+
+@dataclass(kw_only=True)
+class BaseEntity(ABC):
+    """Base class for all domain entities."""
+
+    id: UUID = field(default_factory=uuid4)
+    created_at: datetime = field(default_factory=lambda: datetime.now(timezone.utc))
+    updated_at: datetime = field(default_factory=lambda: datetime.now(timezone.utc))
+
+    def __eq__(self, other: object) -> bool:
+        if not isinstance(other, BaseEntity):
+            return NotImplemented
+        return self.id == other.id
+
+    def __hash__(self) -> int:
+        return hash(self.id)
+
+    def touch(self) -> None:
+        """Update the updated_at timestamp."""
+        self.updated_at = datetime.now(timezone.utc)
+
+    @abstractmethod
+    def to_dict(self) -> dict[str, Any]:
+        """Convert entity to dictionary."""
+        ...