docs(comments): correct comments the cuts left stale

The registration language is read today (preferences.language, the mail language); the Modal and
the players order against popovers through layerStack, not the FloatingTree; the upload's auth
dependency is current_user; the module of access tests covers onboarding and access, not only the
canonical email. The dev proxy's WebSocket upgrade served the removed messages socket and goes; the
test compose header says a pure-logic run needs SIFTLODE_TEST_REQUIRE_DB=0; the speed measure no
longer reports an unreachable database as "no share link".

backend/docker-compose.test.yml

@@ -1,7 +1,8 @@
 # DB-backed pytest lane: an ephemeral Postgres + the test image, wired together so the
 # `db` fixture (tests/conftest.py) has a real database. The whole thing is throwaway — the pg data
-# lives in tmpfs (RAM) and nothing is persisted. The pure-logic lane still runs WITHOUT this file
-# (plain `docker run … python -m pytest` — DB tests skip when no Postgres is reachable).
+# lives in tmpfs (RAM) and nothing is persisted. A pure-logic run without this file is possible but
+# must say so: `docker run -e SIFTLODE_TEST_REQUIRE_DB=0 … python -m pytest` (without it, a missing
+# Postgres fails the run instead of skipping the DB tests).
 #
 #   docker compose -f backend/docker-compose.test.yml up --build --abort-on-container-exit \
 #     --exit-code-from test