Harden staging and bring GitHub/Gitea CI to parity

Address issue #20 and the CI divergence between Gitea and GitHub.

Issue #20 (staging seeded from a prod snapshot):
- Read SECRET_KEY from the environment with the insecure dev key as
  fallback, so each deployment can have its own key.
- Add a `scrub_staging` management command that clears django_session and
  the django-q schedule/queue/results, removing copied prod sessions and
  the inherited convert_prices() schedule.
- Run the scrub from entrypoint.sh when STAGING=true, and wire STAGING plus
  a per-branch SECRET_KEY into the Gitea staging deploy.

CI parity (both systems kept, independent):
- Add the Node/pnpm/TypeScript build steps to the Gitea build workflow to
  match the GitHub test job.
- Add a GitHub staging workflow that deploys per-branch ephemeral instances
  to Fly.io (*.fly.dev) with a fresh database seeded from sample fixtures
  and its own SECRET_KEY, never production data. Tears the app down on
  branch delete and comments the URL on the open PR via github-script.
- Add fly.staging.toml and a LOAD_SAMPLE_DATA entrypoint hook for the
  fresh-database public staging.

https://claude.ai/code/session_01KYjUcNjLfZ8Hq1GAC8J4oZ

games/management/commands/scrub_staging.py

@@ -0,0 +1,28 @@
+from django.contrib.sessions.models import Session
+from django.core.management.base import BaseCommand
+from django_q.models import OrmQ, Schedule, Task
+
+
+class Command(BaseCommand):
+    help = (
+        "Remove copied production artifacts from a staging database seeded "
+        "from a production snapshot: clears authenticated sessions and the "
+        "django-q schedule/queue/results so staging does not share prod's "
+        "session cookies or independently run scheduled tasks."
+    )
+
+    def handle(self, *args, **kwargs):
+        sessions_deleted, _ = Session.objects.all().delete()
+        schedules_deleted, _ = Schedule.objects.all().delete()
+        tasks_deleted, _ = Task.objects.all().delete()
+        queued_deleted, _ = OrmQ.objects.all().delete()
+
+        self.stdout.write(
+            self.style.SUCCESS(
+                "Scrubbed staging database: "
+                f"{sessions_deleted} session(s), "
+                f"{schedules_deleted} schedule(s), "
+                f"{tasks_deleted} task result(s), "
+                f"{queued_deleted} queued task(s) removed."
+            )
+        )