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/value_objects/title.py

@@ -0,0 +1,23 @@
+"""Title value object."""
+
+from dataclasses import dataclass
+
+from app.domain.value_objects.base import ValueObject
+
+
+@dataclass(frozen=True, slots=True)
+class Title(ValueObject[str]):
+    """Blog post title value object."""
+
+    MIN_LENGTH: int = 3
+    MAX_LENGTH: int = 200
+
+    def _validate(self) -> None:
+        if not isinstance(self.value, str):
+            raise ValueError("Title must be a string")
+        if len(self.value) < self.MIN_LENGTH:
+            raise ValueError(f"Title must be at least {self.MIN_LENGTH} characters")
+        if len(self.value) > self.MAX_LENGTH:
+            raise ValueError(f"Title must be at most {self.MAX_LENGTH} characters")
+        if not self.value.strip():
+            raise ValueError("Title cannot be empty or whitespace")