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_guest_cleanup.py

@@ -1,7 +1,8 @@
-"""Guest retention policy — app/cleanup.py.
+"""Guest retention policy: app/cleanup.py.
 
-Covers the two things that matter: that idle guests and their whole data
-graph actually go, and that nothing else ever does.
+These tests cover the two things that matter. Idle guests and their whole
+data graph must actually be deleted, and nothing else must ever be
+deleted.
 
     python -m pytest tests/test_guest_cleanup.py -v
 """
@@ -30,8 +31,8 @@ def db(tmp_path):
 
     @event.listens_for(engine, "connect")
     def _fk(dbapi_connection, _record):
-        # The whole policy leans on ON DELETE CASCADE; SQLite ignores every
-        # one of them unless this is set (same as database.py does).
+        # The whole policy relies on ON DELETE CASCADE. SQLite ignores every
+        # one of them unless this is set, the same as database.py does.
         cur = dbapi_connection.cursor()
         cur.execute("PRAGMA foreign_keys=ON")
         cur.close()
@@ -86,7 +87,7 @@ def test_keeps_guest_inside_the_window(db):
 
 
 def test_boundary_is_not_yet_stale(db):
-    # Exactly 5 days survives; the comparison is strict.
+    # Exactly 5 days survives. The comparison is strict.
     user = make_user(db, days_idle=cleanup.RETENTION_DAYS)
     assert sweep(db) == 0
     assert alive(db, user.id)
@@ -112,8 +113,9 @@ def test_recent_visit_beats_an_old_created_at(db):
 # ---------- what must never go ----------
 
 def test_spares_registered_users(db):
-    """Registering upgrades the guest row in place, so an idle account here is
-    a real user with real data — the whole point of signing up."""
+    """Registering upgrades the guest row in place. An idle account here is
+    a real user with real data, which is what signing up is meant to
+    protect."""
     user = make_user(db, days_idle=400, guest=False, email="a@b.com")
     assert sweep(db) == 0
     assert alive(db, user.id)
@@ -127,7 +129,7 @@ def test_spares_the_local_mode_user(db):
 
 
 def test_spares_a_guest_flagged_row_that_has_an_email(db):
-    # Shouldn't exist, but both clauses are checked so it can't be collected.
+    # This row should not exist, but both clauses are checked so it cannot be collected.
     user = make_user(db, days_idle=400, guest=True, email="odd@b.com")
     assert sweep(db) == 0
     assert alive(db, user.id)
@@ -164,10 +166,10 @@ def test_enabled_requires_multi_user(monkeypatch):
 # ---------- the cascade ----------
 
 def test_deletes_the_whole_data_graph(db):
-    """One DELETE has to take the adventure, its actions and memories, the
-    story cards and the settings row with it — nothing is loaded into Python,
-    so if the FK cascade isn't reaching, rows are silently orphaned (or the
-    statement errors) rather than tidied."""
+    """One DELETE must remove the adventure, its actions and memories, the
+    story cards, and the settings row. Nothing is loaded into Python, so if
+    the FK cascade does not reach a table, its rows are silently orphaned,
+    or the statement fails, instead of being removed."""
     user = make_user(db, days_idle=30)
     scenario = models.Scenario(user_id=user.id, title="S")
     db.add(scenario)