ci: rebuild flex 3.23 image nightly (#687)

## Summary

Adds a `schedule` trigger to `.github/workflows/build-323.yml` so the
default `openemr/openemr:flex` tag refreshes nightly at 2 AM UTC,
matching the existing pattern in `build-edge.yml`, `build-810.yml`, and
`build-811.yml`.

Without this, fixes that land on `master` (devops or upstream openemr)
can sit in the unpublished `openemr/openemr:flex` image for weeks until
someone manually dispatches the workflow. The motivating example: #666
merged on 2026-05-01, but the published `flex` digest is still the
2026-04-07 build, so `openemr-cmd dev-reset-install-demodata` continues
to fail in fresh containers.

When a future Alpine version takes over `is_default_flex: true`, this
trigger should move with it.

Closes #686

## Test plan

- [ ] Verify actionlint passes
- [ ] After merge, confirm a scheduled run kicks off the next 2 AM UTC
and publishes a fresh `openemr/openemr:flex` digest

Author: kojiromike

.github/workflows/build-323.yml

@@ -17,6 +17,8 @@ name: Flex 3.23 Dockers Build
 
 on:
   workflow_dispatch:
+  schedule:
+    - cron: '0 2 * * *' # run at 2 AM UTC
 
 jobs:
   build: