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>

db.py

@@ -1,3 +1,5 @@
+"""SQLAlchemy engine and session factory setup."""
+
 from sqlalchemy import create_engine
 from sqlalchemy.orm import sessionmaker
 
@@ -7,12 +9,19 @@ DB_PATH = "sqlite:///wordsearch.db"
 
 
 def get_engine(db_url=None):
+    """Create a SQLAlchemy engine.
+
+    Args:
+        db_url: Database URL. Defaults to ``sqlite:///wordsearch.db``.
+    """
     return create_engine(db_url or DB_PATH)
 
 
 def get_session_factory(engine):
+    """Return a sessionmaker bound to the given engine."""
     return sessionmaker(bind=engine)
 
 
 def init_db(engine):
+    """Create all tables defined in the ORM models."""
     Base.metadata.create_all(engine)