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/__init__.py

@@ -0,0 +1,34 @@
+"""Domain layer exports."""
+
+from app.domain.entities import BaseEntity, Post
+from app.domain.exceptions import (
+    AlreadyExistsException,
+    DomainException,
+    ForbiddenException,
+    NotFoundException,
+    UnauthorizedException,
+    ValidationException,
+)
+from app.domain.repositories import PostRepository, Repository
+from app.domain.value_objects import Content, Slug, Title, ValueObject
+
+__all__ = [
+    # Entities
+    "BaseEntity",
+    "Post",
+    # Value Objects
+    "ValueObject",
+    "Title",
+    "Content",
+    "Slug",
+    # Repositories
+    "Repository",
+    "PostRepository",
+    # Exceptions
+    "DomainException",
+    "ValidationException",
+    "NotFoundException",
+    "AlreadyExistsException",
+    "UnauthorizedException",
+    "ForbiddenException",
+]