Run staging on a single Fly machine

Staging stores sessions in machine-local SQLite with no shared volume.
Fly's default deploy provisions two machines (HA), so requests after
login could land on the machine that never wrote the session row,
bouncing logged-in users straight back to the login page.

Deploy with --ha=false and scale count 1 so each per-branch staging app
runs on exactly one machine.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YWAvjVEAibhwbeVgbYmg94

.github/workflows/staging.yml

@@ -49,7 +49,13 @@ jobs:
             "APP_URL=https://${HOST}"
 
       - name: Deploy
-        run: flyctl deploy --app "$APP" --config fly.staging.toml --remote-only --yes
+        # --ha=false so Fly provisions a single machine. Staging stores sessions
+        # in machine-local SQLite (no shared volume), so a second machine would
+        # serve requests it has no session for, bouncing logged-in users back to
+        # the login page. scale count 1 fixes apps already created with two.
+        run: |
+          flyctl deploy --app "$APP" --config fly.staging.toml --remote-only --yes --ha=false
+          flyctl scale count 1 --app "$APP" --yes
 
       - name: Summary
         run: echo "Deployed to https://${HOST}" >> "$GITHUB_STEP_SUMMARY"