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/schemas/post.py

@@ -0,0 +1,66 @@
+"""API schemas for posts."""
+
+from datetime import datetime
+from uuid import UUID
+
+from pydantic import BaseModel, ConfigDict, Field
+
+
+class PostBaseSchema(BaseModel):
+    """Base schema for posts."""
+
+    model_config = ConfigDict(from_attributes=True)
+
+    title: str = Field(..., min_length=3, max_length=200)
+    content: str = Field(..., min_length=10, max_length=50000)
+
+
+class PostCreateSchema(PostBaseSchema):
+    """Schema for creating a post."""
+
+    tags: list[str] = Field(default_factory=list)
+
+
+class PostUpdateSchema(BaseModel):
+    """Schema for updating a post."""
+
+    model_config = ConfigDict(from_attributes=True)
+
+    title: str | None = Field(None, min_length=3, max_length=200)
+    content: str | None = Field(None, min_length=10, max_length=50000)
+    tags: list[str] | None = None
+
+
+class PostResponseSchema(BaseModel):
+    """Schema for post response."""
+
+    model_config = ConfigDict(from_attributes=True)
+
+    id: UUID
+    title: str
+    content: str
+    slug: str
+    author_id: str
+    published: bool
+    tags: list[str]
+    created_at: datetime
+    updated_at: datetime
+
+
+class PostListResponseSchema(BaseModel):
+    """Schema for list of posts response."""
+
+    items: list[PostResponseSchema]
+    total: int
+
+
+class PostSearchSchema(BaseModel):
+    """Schema for searching posts."""
+
+    query: str = Field(..., min_length=1, max_length=100)
+
+
+class PostPublishSchema(BaseModel):
+    """Schema for publishing/unpublishing a post."""
+
+    published: bool