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/routers/scenarios.py

@@ -54,9 +54,10 @@ def get_scenario(
     user: models.User = Depends(auth.get_current_user),
 ):
     scenario = get_scenario_or_404(scenario_id, db, user)
-    # Funnel step. Only for shared scenarios: opening one is the first sign a
-    # visitor is interested, whereas someone editing their own is already past
-    # this point — and their titles are theirs, not a statistic.
+    # A funnel step, recorded for shared scenarios only. Opening one is the
+    # first sign that a visitor is interested, and someone editing their own
+    # scenario is already past this point. Their titles are theirs rather than a
+    # statistic.
     if scenario.is_public:
         analytics.record_event(analytics.EV_SCENARIO_OPEN, user)
     return scenario
@@ -204,8 +205,9 @@ def import_scenario(
     if isinstance(schema, dict):
         fields["stat_schema"] = schema
 
-    # AI Dungeon bundles carry an `image` too, so this is worth honouring — but
-    # it's untrusted input, hence sanitize() rather than a straight assignment.
+    # An AI Dungeon bundle also carries an `image`, so this reads it. The value
+    # is untrusted input, so it goes through `sanitize()` rather than a direct
+    # assignment.
     image = images.sanitize(bundle.get("image"), schemas.IMAGE_MAX)
     if image:
         fields["image"] = image
@@ -216,10 +218,10 @@ def import_scenario(
     scenario = models.Scenario(**fields, user_id=user.id)
     if not scenario.title:
         scenario.title = "Imported Scenario"
-    # Raw-dict import bypasses the schemas — clamp to VARCHAR widths
-    # (Postgres enforces them; see schemas.py). Column defaults haven't been
-    # applied yet at this point (that happens at flush), so a bundle with no
-    # `tags` key leaves the attribute None — hence the `or ""`.
+    # A raw-dict import bypasses the schemas, so truncate to the VARCHAR widths.
+    # Postgres enforces them. See `schemas.py`. Column defaults have not been
+    # applied yet, because that happens at flush, so a bundle with no `tags` key
+    # leaves the attribute None, which is why the code says `or ""`.
     scenario.title = scenario.title[:schemas.NAME_MAX]
     scenario.tags = (scenario.tags or "")[:schemas.TAGS_MAX]
     db.add(scenario)