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

tests/unit/conftest.py

@@ -1,18 +1,29 @@
-# Unit test fixtures
-# Provides: mocks, stubs, isolated test data
+"""Unit test fixtures."""
 
 from unittest.mock import AsyncMock, Mock
 
 import pytest
 
+from app.application.interfaces import TransactionManager
+from app.domain.repositories import PostRepository
+
 
 @pytest.fixture
-def mock_service() -> Mock:
-    """Create a mock service for unit testing."""
-    return Mock()
+def mock_post_repository() -> Mock:
+    """Create a mock post repository."""
+    return Mock(spec=PostRepository)
+
+
+@pytest.fixture
+def mock_transaction_manager() -> Mock:
+    """Create a mock transaction manager."""
+    tx_manager = Mock(spec=TransactionManager)
+    tx_manager.commit = AsyncMock()
+    tx_manager.rollback = AsyncMock()
+    return tx_manager
 
 
 @pytest.fixture
 def mock_async_service() -> AsyncMock:
-    """Create an async mock service for unit testing."""
+    """Create an async mock service."""
     return AsyncMock()