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/slug.py

@@ -0,0 +1,41 @@
+"""Slug value object for URL-friendly identifiers."""
+
+import re
+from dataclasses import dataclass
+
+from app.domain.value_objects.base import ValueObject
+
+
+@dataclass(frozen=True, slots=True)
+class Slug(ValueObject[str]):
+    """URL slug value object."""
+
+    MAX_LENGTH: int = 200
+    SLUG_PATTERN: str = r"^[a-z0-9]+(?:-[a-z0-9]+)*$"
+
+    def _validate(self) -> None:
+        if not isinstance(self.value, str):
+            raise ValueError("Slug must be a string")
+        if len(self.value) > self.MAX_LENGTH:
+            raise ValueError(f"Slug must be at most {self.MAX_LENGTH} characters")
+        if not re.match(self.SLUG_PATTERN, self.value):
+            raise ValueError(
+                "Slug must contain only lowercase letters, numbers, and hyphens"
+            )
+
+    @classmethod
+    def from_title(cls, title: str) -> "Slug":
+        """Generate slug from title."""
+        # Convert to lowercase, replace spaces with hyphens
+        slug = title.lower().strip()
+        # Keep only alphanumeric, spaces, and hyphens
+        slug = re.sub(r"[^a-z0-9\s-]", "", slug)
+        # Replace spaces and multiple hyphens with single hyphen
+        slug = re.sub(r"[-\s]+", "-", slug)
+        # Limit length and strip hyphens
+        max_len = 200  # Same as MAX_LENGTH
+        slug = slug[:max_len].strip("-")
+        # Ensure we have at least one character
+        if not slug:
+            slug = "post"
+        return cls(value=slug)