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

tests/unit/domain/test_exceptions.py

@@ -0,0 +1,48 @@
+"""Tests for domain exceptions."""
+
+from app.domain.exceptions import (
+    AlreadyExistsException,
+    DomainException,
+    ForbiddenException,
+    NotFoundException,
+    UnauthorizedException,
+    ValidationException,
+)
+
+
+class TestDomainExceptions:
+    def test_base_exception(self) -> None:
+        """Test base domain exception."""
+        exc = DomainException("Something went wrong")
+        assert exc.message == "Something went wrong"
+        assert str(exc) == "Something went wrong"
+
+    def test_validation_exception(self) -> None:
+        """Test validation exception."""
+        exc = ValidationException("Invalid input")
+        assert isinstance(exc, DomainException)
+        assert exc.message == "Invalid input"
+
+    def test_not_found_exception(self) -> None:
+        """Test not found exception."""
+        exc = NotFoundException("Resource not found")
+        assert isinstance(exc, DomainException)
+        assert exc.message == "Resource not found"
+
+    def test_already_exists_exception(self) -> None:
+        """Test already exists exception."""
+        exc = AlreadyExistsException("Already exists")
+        assert isinstance(exc, DomainException)
+        assert exc.message == "Already exists"
+
+    def test_unauthorized_exception(self) -> None:
+        """Test unauthorized exception."""
+        exc = UnauthorizedException("Unauthorized")
+        assert isinstance(exc, DomainException)
+        assert exc.message == "Unauthorized"
+
+    def test_forbidden_exception(self) -> None:
+        """Test forbidden exception."""
+        exc = ForbiddenException("Forbidden")
+        assert isinstance(exc, DomainException)
+        assert exc.message == "Forbidden"