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/presentation/api/deps.py

@@ -0,0 +1,34 @@
+"""API dependencies using Dishka."""
+
+from typing import Annotated
+
+from dishka.integrations.fastapi import FromDishka
+from fastapi import Depends, Header
+
+from app.application import (
+    CreatePostUseCase,
+    DeletePostUseCase,
+    GetPostUseCase,
+    ListPostsUseCase,
+    PublishPostUseCase,
+    UpdatePostUseCase,
+)
+
+# Use case dependencies - injected via Dishka
+CreatePostDep = FromDishka[CreatePostUseCase]
+GetPostDep = FromDishka[GetPostUseCase]
+UpdatePostDep = FromDishka[UpdatePostUseCase]
+DeletePostDep = FromDishka[DeletePostUseCase]
+ListPostsDep = FromDishka[ListPostsUseCase]
+PublishPostDep = FromDishka[PublishPostUseCase]
+
+
+# Mock current user dependency (replace with real auth)
+async def get_current_user_id(
+    x_user_id: Annotated[str | None, Header()] = "user-123",
+) -> str:
+    """Get current user ID from header."""
+    return x_user_id or "user-123"
+
+
+CurrentUserDep = Annotated[str, Depends(get_current_user_id)]