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/di/transaction_manager.py

@@ -1,4 +1,8 @@
-"""SQLAlchemy implementation of Transaction Manager."""
+"""SQLAlchemy implementation of Transaction Manager.
+
+This module provides the concrete implementation of TransactionManager
+using SQLAlchemy async sessions for transaction control.
+"""
 
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -6,19 +10,44 @@ from app.application.interfaces import TransactionManager
 
 
 class SessionTransactionManager(TransactionManager):
-    """SQLAlchemy Session-based Transaction Manager."""
+    """SQLAlchemy Session-based Transaction Manager.
+
+    Implements transaction control using SQLAlchemy async session.
+    Tracks commit state to prevent duplicate commits.
+
+    Attributes:
+        _session: SQLAlchemy async session for transactions.
+        _committed: Flag indicating if transaction was committed.
+
+    Example:
+        >>> tx_manager = SessionTransactionManager(session)
+        >>> await tx_manager.commit()
+    """
 
     def __init__(self, session: AsyncSession) -> None:
+        """Initialize transaction manager.
+
+        Args:
+            session: SQLAlchemy async session instance.
+        """
         self._session = session
         self._committed: bool = False
 
     async def commit(self) -> None:
-        """Commit the current transaction."""
+        """Commit the current transaction.
+
+        Persists all pending changes to the database.
+        Only commits once - subsequent calls are no-ops.
+        """
         if not self._committed:
             await self._session.commit()
             self._committed = True
 
     async def rollback(self) -> None:
-        """Rollback the current transaction."""
+        """Rollback the current transaction.
+
+        Discards all pending changes.
+        Only rolls back if not already committed.
+        """
         if not self._committed:
             await self._session.rollback()