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/repositories/post.py

@@ -0,0 +1,40 @@
+"""Post repository interface."""
+
+from abc import abstractmethod
+
+from app.domain.entities.post import Post
+from app.domain.repositories.base import Repository
+
+
+class PostRepository(Repository[Post]):
+    """Repository interface for Blog Posts."""
+
+    @abstractmethod
+    async def get_by_slug(self, slug: str) -> Post | None:
+        """Get post by slug."""
+        ...
+
+    @abstractmethod
+    async def get_by_author(self, author_id: str) -> list[Post]:
+        """Get all posts by author."""
+        ...
+
+    @abstractmethod
+    async def get_published(self) -> list[Post]:
+        """Get all published posts."""
+        ...
+
+    @abstractmethod
+    async def get_by_tag(self, tag: str) -> list[Post]:
+        """Get posts by tag."""
+        ...
+
+    @abstractmethod
+    async def slug_exists(self, slug: str) -> bool:
+        """Check if slug already exists."""
+        ...
+
+    @abstractmethod
+    async def search(self, query: str) -> list[Post]:
+        """Search posts by query string."""
+        ...