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

AGENTS.md

@@ -2,6 +2,7 @@
 
 ## Stack
 - Python 3.13+, FastAPI, pydantic, uvicorn
+- SQLAlchemy 2.0 (async), aiosqlite
 - Package manager: `uv`
 - CI: Woodpecker (lint, test, type on push/PR to `dev`)
 
@@ -20,25 +21,124 @@ uv run blog                 # Start dev server (port 8000)
 ## Pre-commit order
 `ruff check --fix` → `ruff format` → `isort` → `mypy`
 
-## Architecture
+## DDD Architecture
+
+### Layer Structure
 ```
 app/
-  main.py          # Entry point, uvicorn.run(app_factory)
-  core/config.py   # Settings from .env via pydantic-settings
-  core/exceptions.py
-  common/error_handler.py
-  api/v1/
-  modules/
+├── domain/                   # Domain Layer - business logic, no dependencies
+│   ├── entities/            # Domain entities (Post, User, etc.)
+│   │   ├── base.py          # Base entity class
+│   │   └── post.py          # Post entity with business logic
+│   ├── value_objects/       # Value objects (Title, Content, Slug)
+│   │   ├── base.py
+│   │   ├── title.py
+│   │   ├── content.py
+│   │   └── slug.py
+│   ├── repositories/        # Repository interfaces (abstract)
+│   │   ├── base.py
+│   │   └── post.py
+│   └── exceptions.py        # Domain exceptions
+│
+├── application/             # Application Layer - use cases
+│   ├── dtos/               # Data Transfer Objects
+│   │   └── post.py
+│   ├── interfaces/         # Abstract interfaces (UoW)
+│   │   └── unit_of_work.py
+│   └── use_cases/          # Use cases (CQRS-like)
+│       ├── create_post.py
+│       ├── get_post.py
+│       ├── update_post.py
+│       ├── delete_post.py
+│       ├── list_posts.py
+│       └── publish_post.py
+│
+├── infrastructure/          # Infrastructure Layer - external concerns
+│   ├── config/             # Configuration
+│   │   └── settings.py
+│   ├── database/           # Database connection & ORM models
+│   │   ├── connection.py
+│   │   └── models.py
+│   ├── repositories/       # Repository implementations
+│   │   ├── post.py         # SQLAlchemyPostRepository
+│   │   └── unit_of_work.py # SQLAlchemyUnitOfWork
+│   ├── di/                 # Dependency Injection
+│   │   └── container.py
+│   └── middleware/         # Exception handlers
+│       └── error_handler.py
+│
+├── presentation/            # Presentation Layer - API
+│   ├── api/                # FastAPI routes
+│   │   ├── v1/            # API version 1
+│   │   │   ├── __init__.py
+│   │   │   └── posts.py   # Posts endpoints
+│   │   ├── deps.py        # FastAPI dependencies
+│   │   └── __init__.py
+│   └── schemas/           # Pydantic schemas
+│       └── post.py
+│
+└── main.py                # Application entry point
+
 tests/
-  unit/
-  integration/
-  e2e/
-  api/
+├── unit/                  # Unit tests (domain, use cases)
+│   ├── domain/           # Domain layer tests
+│   ├── application/      # Application layer tests
+│   └── infrastructure/   # Infrastructure tests
+├── integration/          # Integration tests (DB, repos)
+├── api/                  # API endpoint tests
+└── e2e/                  # End-to-end tests
 ```
 
-## Key conventions
-- All commands use `uv run` prefix
-- pytest: asyncio_mode=auto, coverage on `app/`
-- mypy: strict=true with pydantic plugin
-- isort: black profile, filter_files=true
-- `.env` loaded by pydantic-settings (not in repo)
+## Key Conventions
+
+### Dependency Rule
+- Domain layer has **NO dependencies** on other layers
+- Application layer depends only on Domain
+- Infrastructure depends on Domain and Application
+- Presentation depends on all other layers
+
+### Testing
+- **Unit tests**: Test domain logic without DB/external services
+- **Integration tests**: Test repository implementations with real DB
+- **API tests**: Test endpoints with mocked use cases
+- **E2E tests**: Full workflow testing
+
+### Code Patterns
+- Use **dataclasses** for entities and value objects
+- Use **frozen dataclasses** for value objects (immutable)
+- Use **Unit of Work** pattern for transactions
+- Use **Repository** pattern for data access
+- Use **Dependency Injection** via FastAPI's Depends()
+
+## DDD Concepts Used
+
+### Entities
+- Have identity (UUID)
+- Mutable state
+- Business logic methods (publish, update_title, etc.)
+- Example: `Post` entity
+
+### Value Objects
+- Immutable
+- Defined by attributes
+- Validated on creation
+- Examples: `Title`, `Content`, `Slug`
+
+### Aggregates & Repositories
+- `Post` is an aggregate root
+- `PostRepository` interface in Domain
+- `SQLAlchemyPostRepository` implementation in Infrastructure
+
+### Domain Events
+- Placeholder for future implementation
+- Can be added via event bus in application layer
+
+## Configuration
+- `.env` file loaded by pydantic-settings
+- Settings available via `app.infrastructure.config.settings`
+
+## Database
+- SQLAlchemy 2.0 with async support
+- SQLite by default (aiosqlite)
+- Tables auto-created on startup
+- Use `init_db()` and `close_db()` in lifespan