Add Beam scheduler app for CapRover

[fmercurio]

Summary

• add a dedicated, non-public st-remp-beam-scheduler CapRover app
• run Laravel schedule:work from the exact immutable Beam image already pinned in staging
• validate all Captain definitions and shell entrypoints in PR CI
• document why the scheduler is required for pageview/timespent aggregation and dashboard refresh

Runtime evidence

The tracker accepted synthetic pageview payloads with HTTP 202 and general events reached the Beam dashboard, but article resources remained at pageviews_all=0 and timespent_all=0. The deployed Beam image starts only PHP-FPM, while Beam/extensions/beam-module/src/Scheduler.php defines per-minute aggregation, snapshots, and dashboard refresh jobs. No scheduler process is currently deployed.

Safety

• staging only
• no public hostname
• one scheduler instance
• immutable Beam image tag
• no change to the existing Beam PHP-FPM app

Local validation

• python3 caprover/scripts/validate-definitions.py
• bash -n caprover/scripts/*.sh
• workflow YAML parsed successfully
• git diff --check