docs(drupal-core): fix seed cache description and service setup instructions

[rfay]

Summary

Fixes inaccurate and incomplete seed cache documentation introduced in #145, plus a service file bug that prevents the timer from working.

Description fixes:

• Replaced incorrect "database is always installed fresh via ddev drush si" — the seed cache has no DDEV project, no database, no vendor files; it is a simple git checkout
• Fixed ddev composer install → composer install in the cache-hit description

Migration step fixes (the steps in #145 were incomplete — the timer never ran):

• Add install of drupal-cache-updater.timer (was missing entirely)
• Add sed to substitute User=YOURUSER placeholder in the service file (without this, the service fails to start)
• Add systemctl enable --now drupal-cache-updater.timer (without this, the hourly update is never scheduled)

Service file fix:

• WorkingDirectory=%h → WorkingDirectory=/tmp — %h causes "Permission denied" when the timer fires. The script uses only absolute paths so the working directory is irrelevant.

Script fix:

• Add reinstall reminder to update-drupal-cache so the deployed /usr/local/bin/ copy must be updated manually after script changes

Required admin action on each server

Run these steps to fix the installed service and get the timer working:

REPO=~/workspace/coder-ddev

# Reinstall the updated script and service file
sudo install -m 755 $REPO/drupal-core/scripts/update-drupal-cache \
  /usr/local/bin/update-drupal-cache
sudo install -m 644 $REPO/drupal-core/scripts/drupal-cache-updater.service \
  /etc/systemd/system/
sudo install -m 644 $REPO/drupal-core/scripts/drupal-cache-updater.timer \
  /etc/systemd/system/

# Fix User= placeholder if not already done
sudo grep -q YOURUSER /etc/systemd/system/drupal-cache-updater.service && \
  sudo sed -i "s/User=YOURUSER/User=$(whoami)/" /etc/systemd/system/drupal-cache-updater.service

# Reload and restart
sudo systemctl daemon-reload
sudo systemctl enable --now drupal-cache-updater.timer
sudo systemctl start drupal-cache-updater.service

# Verify
systemctl list-timers drupal-cache-updater.timer
journalctl -u drupal-cache-updater.service --no-pager | tail -10

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