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/application/dtos/post.py

@@ -0,0 +1,39 @@
+"""DTOs for post use cases."""
+
+from dataclasses import dataclass
+from datetime import datetime
+from uuid import UUID
+
+
+@dataclass(frozen=True)
+class CreatePostDTO:
+    """DTO for creating a post."""
+
+    title: str
+    content: str
+    author_id: str
+    tags: list[str] | None = None
+
+
+@dataclass(frozen=True)
+class UpdatePostDTO:
+    """DTO for updating a post."""
+
+    title: str | None = None
+    content: str | None = None
+    tags: list[str] | None = None
+
+
+@dataclass(frozen=True)
+class PostResponseDTO:
+    """DTO for post response."""
+
+    id: UUID
+    title: str
+    content: str
+    slug: str
+    author_id: str
+    published: bool
+    tags: list[str]
+    created_at: datetime
+    updated_at: datetime