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/entities/post.py

@@ -0,0 +1,88 @@
+"""Domain entity for Blog Post."""
+
+from dataclasses import dataclass, field
+from typing import Any
+
+from app.domain.entities.base import BaseEntity
+from app.domain.value_objects.content import Content
+from app.domain.value_objects.slug import Slug
+from app.domain.value_objects.title import Title
+
+
+@dataclass(kw_only=True)
+class Post(BaseEntity):
+    """Blog post domain entity."""
+
+    title: Title
+    content: Content
+    slug: Slug
+    author_id: str
+    published: bool = False
+    tags: list[str] = field(default_factory=list)
+
+    def publish(self) -> None:
+        """Publish the post."""
+        self.published = True
+        self.touch()
+
+    def unpublish(self) -> None:
+        """Unpublish the post."""
+        self.published = False
+        self.touch()
+
+    def update_content(self, content: Content) -> None:
+        """Update post content."""
+        self.content = content
+        self.touch()
+
+    def update_title(self, title: Title) -> None:
+        """Update post title and regenerate slug."""
+        self.title = title
+        self.slug = Slug.from_title(title.value)
+        self.touch()
+
+    def add_tag(self, tag: str) -> None:
+        """Add a tag to the post."""
+        if tag not in self.tags:
+            self.tags.append(tag)
+            self.touch()
+
+    def remove_tag(self, tag: str) -> None:
+        """Remove a tag from the post."""
+        if tag in self.tags:
+            self.tags.remove(tag)
+            self.touch()
+
+    def to_dict(self) -> dict[str, Any]:
+        """Convert entity to dictionary."""
+        return {
+            "id": str(self.id),
+            "title": self.title.value,
+            "content": self.content.value,
+            "slug": self.slug.value,
+            "author_id": self.author_id,
+            "published": self.published,
+            "tags": self.tags.copy(),
+            "created_at": self.created_at.isoformat(),
+            "updated_at": self.updated_at.isoformat(),
+        }
+
+    @classmethod
+    def create(
+        cls,
+        title_str: str,
+        content_str: str,
+        author_id: str,
+        tags: list[str] | None = None,
+    ) -> "Post":
+        """Factory method to create a new post."""
+        title = Title(title_str)
+        content = Content(content_str)
+        slug = Slug.from_title(title_str)
+        return cls(
+            title=title,
+            content=content,
+            slug=slug,
+            author_id=author_id,
+            tags=tags or [],
+        )