Add daily systemd timer for OG image generation

[retlehs]

Summary

• The generate-og command existed but was never scheduled — OG images were only generated on first page view and never updated
• Adds a daily systemd timer (3am) to regenerate OG images where install counts have changed since last generation
• Updates operations docs with the new scheduled task
• Fixes misleading doc comment on GetPackagesNeedingOG (claimed to detect metadata changes but only checks install counts)

Deploy notes

• Run the provision playbook to deploy the new systemd service + timer units
• After provisioning, manually start the timer for the first time: sudo systemctl start wpcomposer-generate-og.timer

Test plan

• Verify wpcomposer generate-og runs successfully on the server
• Confirm timer is active: systemctl status wpcomposer-generate-og.timer
• After 24h, verify updated OG images appear in R2 for packages with changed install counts

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