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/database/models.py

@@ -0,0 +1,40 @@
+"""SQLAlchemy ORM models."""
+
+from datetime import datetime, timezone
+from uuid import uuid4
+
+from sqlalchemy import JSON, Boolean, DateTime, String, Text
+from sqlalchemy.orm import Mapped, declarative_base, mapped_column
+
+Base = declarative_base()
+
+
+class PostORM(Base):  # type: ignore[valid-type,misc]
+    """SQLAlchemy model for Blog Post."""
+
+    __tablename__ = "posts"
+
+    id: Mapped[str] = mapped_column(
+        String(36), primary_key=True, default=lambda: str(uuid4())
+    )
+    title: Mapped[str] = mapped_column(String(200), nullable=False)
+    content: Mapped[str] = mapped_column(Text, nullable=False)
+    slug: Mapped[str] = mapped_column(
+        String(200), nullable=False, unique=True, index=True
+    )
+    author_id: Mapped[str] = mapped_column(String(100), nullable=False, index=True)
+    published: Mapped[bool] = mapped_column(
+        Boolean, default=False, nullable=False, index=True
+    )
+    tags: Mapped[list[str]] = mapped_column(JSON, default=list)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc),
+        nullable=False,
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc),
+        onupdate=lambda: datetime.now(timezone.utc),
+        nullable=False,
+    )