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>

game/factory.py

@@ -1,3 +1,5 @@
+"""Board generation for word search puzzles."""
+
 import random
 import string
 
@@ -19,10 +21,29 @@ MAX_PLACEMENT_ATTEMPTS = 100
 
 
 class BoardFactory:
+    """Generates word search boards from category word lists.
+
+    Args:
+        session: SQLAlchemy session for querying category words.
+    """
+
     def __init__(self, session):
         self.session = session
 
     def create(self, category_id, board_size=10, word_count=8):
+        """Generate a new game board.
+
+        Selects random words from the category, places them on the grid
+        in random directions, and fills remaining cells with random letters.
+
+        Args:
+            category_id: ID of the category to pull words from.
+            board_size: Grid dimension (N x N).
+            word_count: Maximum number of words to place.
+
+        Returns:
+            Tuple of (Game, list[Word]) with objects not yet committed.
+        """
         candidates = (
             self.session.query(CategoryWord)
             .filter(CategoryWord.category_id == category_id)