docs: add Google-style docstrings and Sphinx autodoc setup

Add concise docstrings to all Python modules, classes, and public
functions. Configure Sphinx with napoleon extension for Google-style
docstring parsing and autodoc pages for each module.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

models/game.py

@@ -1,3 +1,5 @@
+"""Game session and placed word models."""
+
 import uuid
 from datetime import datetime, timezone
 
@@ -16,6 +18,11 @@ from models import Base
 
 
 class Game(Base):
+    """A single word search game session.
+
+    Stores the generated grid, tracks status, and links to placed words.
+    """
+
     __tablename__ = "games"
 
     id = Column(String(36), primary_key=True, default=lambda: str(uuid.uuid4()))
@@ -33,6 +40,8 @@ class Game(Base):
 
 
 class Word(Base):
+    """A word placed on a game board with its position and found status."""
+
     __tablename__ = "words"
 
     id = Column(Integer, primary_key=True, autoincrement=True)