chore(infra): add reloader for secret rollouts

[soohyunme]

Summary

• Add Stakater Reloader as an infrastructure ArgoCD Application.
• Configure Reloader with reloader.reloadStrategy=annotations for ArgoCD-friendly rollouts.
• Opt in event-bingo-backend to restart when event-bingo-backend-runtime-secrets changes.
• Opt in experiment-platform-backend to restart when experiment-platform-backend-secret changes.
• Add per-Application ignoreDifferences for Reloader's live last-reloaded-from pod template annotation.

Why

Backend pods consume runtime Secrets via envFrom.secretRef; Kubernetes does not update environment variables inside existing pods when a Secret changes. Reloader closes that operational gap without broad auto-restart behavior.

Validation

• Confirmed latest Stakater chart repo version: reloader chart 2.2.12.
• Verified changed YAML parses successfully.
• Verified target backend Deployments reference only their runtime Secret via secret.reloader.stakater.com/reload.
• Verified service ArgoCD Applications ignore /spec/template/metadata/annotations/reloader.stakater.com~1last-reloaded-from.
• Ran git diff --cached --check before commit.

Rollout Notes

After merge, sync the infrastructure App first so Reloader is installed, then sync service apps. Future SealedSecret data changes for the opted-in backend Secrets should trigger backend rollouts automatically.