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/list_posts.py

@@ -0,0 +1,57 @@
+"""List posts use case."""
+
+from app.application.dtos.post import PostResponseDTO
+from app.application.interfaces import TransactionManager
+from app.domain.entities import Post
+from app.domain.repositories import PostRepository
+
+
+class ListPostsUseCase:
+    """Use case for listing blog posts with filtering."""
+
+    def __init__(
+        self,
+        post_repo: PostRepository,
+        tx_manager: TransactionManager,
+    ) -> None:
+        self._post_repo = post_repo
+        self._tx_manager = tx_manager
+
+    async def all_posts(self) -> list[PostResponseDTO]:
+        """Get all posts."""
+        posts = await self._post_repo.get_all()
+        return [self._map_to_dto(post) for post in posts]
+
+    async def published_posts(self) -> list[PostResponseDTO]:
+        """Get all published posts."""
+        posts = await self._post_repo.get_published()
+        return [self._map_to_dto(post) for post in posts]
+
+    async def by_author(self, author_id: str) -> list[PostResponseDTO]:
+        """Get posts by author."""
+        posts = await self._post_repo.get_by_author(author_id)
+        return [self._map_to_dto(post) for post in posts]
+
+    async def by_tag(self, tag: str) -> list[PostResponseDTO]:
+        """Get posts by tag."""
+        posts = await self._post_repo.get_by_tag(tag)
+        return [self._map_to_dto(post) for post in posts]
+
+    async def search(self, query: str) -> list[PostResponseDTO]:
+        """Search posts."""
+        posts = await self._post_repo.search(query)
+        return [self._map_to_dto(post) for post in posts]
+
+    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,
+        )