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/use_cases/update_post.py

@@ -0,0 +1,73 @@
+"""Update post use case."""
+
+from uuid import UUID
+
+from app.application.dtos.post import PostResponseDTO, UpdatePostDTO
+from app.application.interfaces import TransactionManager
+from app.domain.entities import Post
+from app.domain.exceptions import ForbiddenException, NotFoundException
+from app.domain.repositories import PostRepository
+from app.domain.value_objects import Content, Title
+
+
+class UpdatePostUseCase:
+    """Use case for updating a blog post."""
+
+    def __init__(
+        self,
+        post_repo: PostRepository,
+        tx_manager: TransactionManager,
+    ) -> None:
+        self._post_repo = post_repo
+        self._tx_manager = tx_manager
+
+    async def execute(
+        self,
+        post_id: UUID,
+        dto: UpdatePostDTO,
+        current_user_id: str,
+    ) -> PostResponseDTO:
+        """Execute the use case."""
+        post = await self._post_repo.get_by_id(post_id)
+        if not post:
+            raise NotFoundException(f"Post with id '{post_id}' not found")
+
+        # Check authorization
+        if post.author_id != current_user_id:
+            raise ForbiddenException("You can only update your own posts")
+
+        # Update fields
+        if dto.title is not None:
+            post.update_title(Title(dto.title))
+
+        if dto.content is not None:
+            post.update_content(Content(dto.content))
+
+        if dto.tags is not None:
+            # Replace all tags
+            for tag in post.tags[:]:
+                post.remove_tag(tag)
+            for tag in dto.tags:
+                post.add_tag(tag)
+
+        # Persist changes
+        await self._post_repo.update(post)
+
+        # Commit transaction
+        await self._tx_manager.commit()
+
+        return self._map_to_dto(post)
+
+    def _map_to_dto(self, post: Post) -> PostResponseDTO:
+        """Map domain entity to response DTO."""
+        return PostResponseDTO(
+            id=post.id,
+            title=post.title.value,
+            content=post.content.value,
+            slug=post.slug.value,
+            author_id=post.author_id,
+            published=post.published,
+            tags=post.tags.copy(),
+            created_at=post.created_at,
+            updated_at=post.updated_at,
+        )