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/api/conftest.py

@@ -1,17 +1,16 @@
-# API test fixtures
-# Provides: httpx.AsyncClient, authentication helpers, test API data
+"""API test fixtures."""
 
 from typing import AsyncGenerator
 
 import pytest
 from httpx import ASGITransport, AsyncClient
 
+from app.main import app_factory
+
 
 @pytest.fixture
 async def client() -> AsyncGenerator[AsyncClient, None]:
     """Create async HTTP client for API testing."""
-    from app.main import app_factory
-
     app = app_factory()
     transport = ASGITransport(app=app)
     async with AsyncClient(transport=transport, base_url="http://test") as ac:
@@ -21,4 +20,4 @@ async def client() -> AsyncGenerator[AsyncClient, None]:
 @pytest.fixture
 def auth_headers() -> dict[str, str]:
     """Return mock authentication headers."""
-    return {"Authorization": "Bearer test_token"}
+    return {"Authorization": "Bearer test_token", "X-User-Id": "user-123"}