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/repositories/base.py

@@ -0,0 +1,43 @@
+"""Base repository interface for DDD."""
+
+from abc import ABC, abstractmethod
+from typing import Generic, TypeVar
+from uuid import UUID
+
+from app.domain.entities.base import BaseEntity
+
+T = TypeVar("T", bound=BaseEntity)
+
+
+class Repository(ABC, Generic[T]):
+    """Generic repository interface."""
+
+    @abstractmethod
+    async def get_by_id(self, entity_id: UUID) -> T | None:
+        """Get entity by ID."""
+        ...
+
+    @abstractmethod
+    async def get_all(self) -> list[T]:
+        """Get all entities."""
+        ...
+
+    @abstractmethod
+    async def add(self, entity: T) -> None:
+        """Add new entity."""
+        ...
+
+    @abstractmethod
+    async def update(self, entity: T) -> None:
+        """Update existing entity."""
+        ...
+
+    @abstractmethod
+    async def delete(self, entity_id: UUID) -> None:
+        """Delete entity by ID."""
+        ...
+
+    @abstractmethod
+    async def exists(self, entity_id: UUID) -> bool:
+        """Check if entity exists."""
+        ...