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/infrastructure/database/__init__.py

@@ -1,4 +1,8 @@
-"""Database infrastructure."""
+"""Database infrastructure.
+
+This module re-exports database connection utilities and ORM models
+for data persistence.
+"""
 
 from app.infrastructure.database.connection import (
     AsyncSessionLocal,