Rewrite Python comments in Google developer documentation style (#12)

* Rewrite comments in Google developer documentation style

Rewrite the comments and docstrings across the backend core modules so they
read plainly. The previous prose was accurate but dense and figurative, which
made it slow to skim.

Applies the Google developer documentation style guide: short sentences, active
voice, present tense, American spelling, and no metaphors, idioms, or
rhetorical asides. Replaces em-dash chains with separate sentences.

backend/tests/test_scenario_art.py

@@ -1,9 +1,10 @@
-"""Scenario cover art + the Continue-card snippet.
+"""Scenario cover art and the Continue-card snippet.
 
-Unit tests for the data-URI handling in app/images.py, then HTTP tests that the
-list endpoints advertise a cacheable `image_url` (never the inline base64), that
-the image route serves real bytes, and that an adventure's snippet comes from
-the latest *narration* rather than the player's last line.
+Unit tests for the data-URI handling in app/images.py. Then HTTP tests
+confirm that the list endpoints advertise a cacheable `image_url` instead
+of the inline base64, that the image route serves real bytes, and that an
+adventure's snippet comes from the latest narration rather than the
+player's last line.
 
     python -m pytest tests/test_scenario_art.py -v
 """
@@ -41,13 +42,14 @@ def test_decode_returns_bytes_and_content_type():
 
 
 def test_decode_tolerates_wrapped_base64():
-    """A hand-pasted URI can carry newlines; b64decode(validate=True) won't."""
+    """A hand-pasted URI can carry newlines. `b64decode(validate=True)` rejects them."""
     wrapped = "data:image/png;base64," + "\n".join(
         base64.b64encode(PNG_BYTES).decode()[i:i + 24] for i in range(0, 100, 24)
     )
-    # Only asserting it doesn't raise and doesn't silently return a partial
-    # decode of a truncated payload — the wrapped prefix here is not the whole
-    # image, so a None result is also acceptable; what matters is no exception.
+    # This test only confirms that the call does not raise and does not
+    # silently return a partial decode of a truncated payload. The wrapped
+    # prefix here is not the whole image, so a None result is also
+    # acceptable. What matters is that no exception occurs.
     images.decode(wrapped)
 
 
@@ -59,7 +61,7 @@ def test_decode_rejects_non_data_uris_and_garbage():
 
 
 def test_decode_rejects_svg():
-    """SVG can carry script and these bytes are served from our own origin."""
+    """SVG can carry script, and the app serves these bytes from its own origin."""
     svg = "data:image/svg+xml;base64," + base64.b64encode(b"<svg/>").decode()
     assert images.decode(svg) is None
 
@@ -151,7 +153,7 @@ def test_scenario_list_advertises_a_url_and_hides_the_base64(client):
 
     pictured = rows["Pictured"]
     assert pictured["image_url"].startswith(f"/api/scenarios/{client.ids['scenario']}/image?v=")
-    # The whole point: a list response must never carry the inline image.
+    # A list response must never carry the inline image.
     assert "image" not in pictured
 
     assert rows["Emoji"]["image_url"] == ""
@@ -180,7 +182,7 @@ def test_single_scenario_still_returns_the_raw_uri_for_editing(client):
 def test_adventure_list_carries_snippet_and_inherited_art(client):
     row = next(r for r in client.get("/api/adventures").json()
                if r["id"] == client.ids["adventure"])
-    # Latest narration, whitespace collapsed — not the player's "I draw my sword."
+    # Latest narration, whitespace collapsed. Not the player's line, "I draw my sword."
     assert row["snippet"] == "Steel rings. The corridor answers."
     assert row["image_url"].startswith(f"/api/scenarios/{client.ids['scenario']}/image?v=")
     assert row["action_count"] == 3