docs(talk): address PR #741 reviewer nits

Comment-only follow-up to surface non-obvious behavior at the call
sites flagged in review:

- server/talk.py: note the `uuid.uuid4().hex` 32-char no-dashes format
  (spreed accepts either form).
- models/talk.py: warn that spreed returns `lastReadMessage: 0` rather
  than `null` for unread rooms, so consumers should compare to ``None``
  rather than rely on truthiness.
- 10-install-spreed-app.sh: document that the `app:install || app:enable`
  fallback also masks unrelated install failures, and limit its use to
  dev fixtures.

No runtime behavior changes; tests unchanged (still 13 unit + 7 integ).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app-hooks/post-installation/10-install-spreed-app.sh

@@ -6,4 +6,9 @@ set -euox pipefail
 # app is not yet installed, so app:install pulls it. On rebuilds where the
 # volume already has it, app:install would fail, so fall through to
 # app:enable.
+#
+# Caveat: this `||` also masks unrelated install failures (network outage,
+# bad version pin, etc.) — they will fall through to app:enable, which
+# will then fail with a clearer "app not found" error. Acceptable for a
+# dev-fixture script; do not copy this pattern into production tooling.
 php /var/www/html/occ app:install spreed || php /var/www/html/occ app:enable spreed