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/category.py

@@ -1,3 +1,5 @@
+"""Category and word list models."""
+
 from sqlalchemy import Column, ForeignKey, Integer, String, UniqueConstraint
 from sqlalchemy.orm import relationship
 
@@ -5,6 +7,8 @@ from models import Base
 
 
 class Category(Base):
+    """A word category such as 'animals' or 'food'."""
+
     __tablename__ = "categories"
 
     id = Column(Integer, primary_key=True, autoincrement=True)
@@ -14,6 +18,8 @@ class Category(Base):
 
 
 class CategoryWord(Base):
+    """A word belonging to a category, used as a candidate for board generation."""
+
     __tablename__ = "category_words"
     __table_args__ = (UniqueConstraint("category_id", "word"),)