Reload the browser on template and static changes in dev

Adds django-browser-reload: runserver already restarts on Python changes, but
the browser had to be refreshed by hand for every template or CSS edit. It also
watches static/, so a Tailwind rebuild now refreshes the page on its own.

Mounted only under DEBUG -- it injects a script into every HTML response and
serves an open event stream, neither of which belongs in production; a test
holds that line. Its endpoint is exempt from RequireMFAMiddleware, otherwise a
not-yet-enrolled staff user has the stream redirected away and live reload dies
on the MFA enrolment page, which is exactly a page we are restyling.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pyproject.toml

@@ -17,6 +17,7 @@ dependencies = [
 [dependency-groups]
 dev = [
     "coverage>=7.15.0",
+    "django-browser-reload>=1.21.0",
     "ruff>=0.15.17",
 ]