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/app/images.py

@@ -22,9 +22,9 @@ DATA_URI_RE = re.compile(
 def public_url(scenario_id: int, image: str, version: object) -> str:
     """The URL a client should load for this scenario's art ("" if none).
 
-    `version` (any object with a stable repr — normally the row's updated_at)
-    becomes a cache-buster, letting the image response be marked immutable
-    while still refreshing the moment the author swaps the picture.
+    `version` is any object with a stable repr, and is normally the row's
+    `updated_at`. It becomes a cache-buster, so the image response can be marked
+    immutable and still refresh as soon as the author replaces the picture.
     """
     if not image:
         return ""
@@ -40,9 +40,9 @@ def public_url(scenario_id: int, image: str, version: object) -> str:
 def sanitize(value: object, max_length: int) -> str:
     """Coerce an untrusted `image` value from an import bundle to a safe one.
 
-    Anything that isn't a supported data URI or an https URL — or that is too
-    large to store — becomes "", so a hostile or merely foreign bundle can't
-    smuggle in a `javascript:` URI or blow past the column cap.
+    A value that is not a supported data URI or an https URL, or that is too
+    large to store, becomes "". A hostile or merely unfamiliar bundle therefore
+    cannot pass in a `javascript:` URI or exceed the column cap.
     """
     if not isinstance(value, str) or not value or len(value) > max_length:
         return ""
@@ -62,5 +62,6 @@ def decode(image: str) -> tuple[bytes, str] | None:
         payload = re.sub(r"\s+", "", match.group(2))
         return base64.b64decode(payload, validate=True), match.group(1).lower()
     except (binascii.Error, ValueError):
-        # Truncated or hand-edited base64 — treat as "no image" rather than 500.
+        # The base64 is truncated or hand-edited. Treat it as no image rather
+        # than return a 500.
         return None