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/infrastructure/database/__init__.py

@@ -0,0 +1,22 @@
+"""Database infrastructure."""
+
+from app.infrastructure.database.connection import (
+    AsyncSessionLocal,
+    close_db,
+    engine,
+    get_session,
+    get_session_context,
+    init_db,
+)
+from app.infrastructure.database.models import Base, PostORM
+
+__all__ = [
+    "Base",
+    "PostORM",
+    "engine",
+    "AsyncSessionLocal",
+    "get_session",
+    "get_session_context",
+    "init_db",
+    "close_db",
+]