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/exceptions.py

@@ -0,0 +1,39 @@
+"""Domain exceptions."""
+
+
+class DomainException(Exception):
+    """Base exception for domain layer."""
+
+    def __init__(self, message: str) -> None:
+        self.message = message
+        super().__init__(self.message)
+
+
+class ValidationException(DomainException):
+    """Raised when validation fails."""
+
+    pass
+
+
+class NotFoundException(DomainException):
+    """Raised when an entity is not found."""
+
+    pass
+
+
+class AlreadyExistsException(DomainException):
+    """Raised when trying to create an entity that already exists."""
+
+    pass
+
+
+class UnauthorizedException(DomainException):
+    """Raised when user is not authorized."""
+
+    pass
+
+
+class ForbiddenException(DomainException):
+    """Raised when access is forbidden."""
+
+    pass