[app] Run cache warmup as an init container instead of postStart (port of deployment#73)

[henzigo]

Context

The php-fpm container warms the application cache in a postStart hook
(phing -S warmup). postStart failures are hard to observe, run concurrently with the
container's main process, and the pod can become Ready before the warmup finished.

Upstream PR shopsys/deployment#73 moves the warmup into a dedicated init container
(same image, same env, shared source-codes volume + domains-urls mount) and drops the
postStart hook.

Proposal

Port the same change into deployment-webserver-php-fpm.yaml:

• add init container warmup: app image, command: ["/var/www/html/phing", "-S", "warmup"],
  mounts source-codes + domains-urls, full backend env (shopsys.appEnv)
• remove the postStart hook from the php-fpm container
• consider the same for the cron pod's inline ./phing warmup (separate decision — cron
  warmup is part of its startup command, see [app] Align cron lifecycle with deployment#72 (Recreate, foreground crond, console lock/watch) #25)

Benefits: warmup failures fail the rollout visibly (init container status), readiness is
truthful, logs are separated.

Tasks

• Warmup init container + postStart removal
• Unit test, golden snapshots, deviations entry
• Verify rollout timing impact (warmup now serializes before containers start)

Reference: shopsys/deployment#73