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_value_objects.py

@@ -0,0 +1,93 @@
+"""Tests for domain value objects."""
+
+import pytest
+
+from app.domain.value_objects import Content, Slug, Title
+
+
+class TestTitle:
+    def test_valid_title(self) -> None:
+        """Test creating a valid title."""
+        title = Title("Valid Title")
+        assert title.value == "Valid Title"
+
+    def test_title_too_short(self) -> None:
+        """Test title that is too short."""
+        with pytest.raises(ValueError, match="at least"):
+            Title("ab")
+
+    def test_title_too_long(self) -> None:
+        """Test title that is too long."""
+        with pytest.raises(ValueError, match="at most"):
+            Title("a" * 201)
+
+    def test_title_empty(self) -> None:
+        """Test empty title."""
+        with pytest.raises(ValueError, match="empty"):
+            Title("   ")
+
+    def test_title_not_string(self) -> None:
+        """Test non-string title."""
+        with pytest.raises(ValueError, match="string"):
+            Title(123)  # type: ignore[arg-type]
+
+
+class TestContent:
+    def test_valid_content(self) -> None:
+        """Test creating valid content."""
+        content = Content("This is valid content with enough characters")
+        assert content.value == "This is valid content with enough characters"
+
+    def test_content_too_short(self) -> None:
+        """Test content that is too short."""
+        with pytest.raises(ValueError, match="at least"):
+            Content("short")
+
+    def test_content_too_long(self) -> None:
+        """Test content that is too long."""
+        with pytest.raises(ValueError, match="at most"):
+            Content("a" * 50001)
+
+    def test_content_empty(self) -> None:
+        """Test empty content."""
+        with pytest.raises(ValueError, match="empty"):
+            Content("   ")
+
+
+class TestSlug:
+    def test_valid_slug(self) -> None:
+        """Test creating a valid slug."""
+        slug = Slug("valid-slug")
+        assert slug.value == "valid-slug"
+
+    def test_slug_from_title(self) -> None:
+        """Test generating slug from title."""
+        slug = Slug.from_title("Hello World Post")
+        assert slug.value == "hello-world-post"
+
+    def test_slug_from_title_with_special_chars(self) -> None:
+        """Test generating slug from title with special characters."""
+        slug = Slug.from_title("Hello, World! Post @#$%")
+        assert slug.value == "hello-world-post"
+
+    def test_slug_from_title_only_special_chars(self) -> None:
+        """Test generating slug from title with only special characters."""
+        slug = Slug.from_title("!@#$%")
+        assert slug.value == "post"
+
+    def test_slug_invalid_chars(self) -> None:
+        """Test slug with invalid characters."""
+        with pytest.raises(ValueError, match="lowercase"):
+            Slug("Invalid_Slug")
+
+    def test_slug_uppercase(self) -> None:
+        """Test slug with uppercase letters."""
+        with pytest.raises(ValueError, match="lowercase"):
+            Slug("Uppercase-Slug")
+
+    def test_slug_equality(self) -> None:
+        """Test slug value equality."""
+        slug1 = Slug("test-slug")
+        slug2 = Slug("test-slug")
+        assert slug1 == slug2
+        assert hash(slug1) == hash(slug2)