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/infrastructure/di/transaction_manager.py

@@ -0,0 +1,24 @@
+"""SQLAlchemy implementation of Transaction Manager."""
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.application.interfaces import TransactionManager
+
+
+class SessionTransactionManager(TransactionManager):
+    """SQLAlchemy Session-based Transaction Manager."""
+
+    def __init__(self, session: AsyncSession) -> None:
+        self._session = session
+        self._committed: bool = False
+
+    async def commit(self) -> None:
+        """Commit the current transaction."""
+        if not self._committed:
+            await self._session.commit()
+            self._committed = True
+
+    async def rollback(self) -> None:
+        """Rollback the current transaction."""
+        if not self._committed:
+            await self._session.rollback()