docs: add AI code generation requirements and comprehensive Google-style docstrings

- Add AI code generation requirements to AGENTS.md
- Add module-level docstrings to all 46 Python modules
- Add detailed Google-style docstrings to all classes and functions
- Remove all inline comments following self-documenting code principle
- Include Args, Returns, Raises sections in function docstrings
- Add Attributes and Examples sections to class docstrings

app/__init__.py

@@ -1 +1,5 @@
-"""Application package."""
+"""Blog API Application.
+
+This package provides a complete blog API implementation following
+Domain-Driven Design principles with FastAPI and SQLAlchemy.
+"""