Document configurable cleanup task schedules

[gantoine]

Part of #161. Companion to rommapp/romm#4957.

Updates the Scheduled Tasks page for the release that ships rommapp/romm#4957:

• The housekeeping tasks (netplay, upload tmp, ZIP cache, sync session cleanup) now ship enabled with an ENABLE_SCHEDULED_CLEANUP_* / SCHEDULED_CLEANUP_*_CRON pair each, replacing the "always on and have no env vars" sentence.
• Turning off upload tmp cleanup leaves abandoned chunked uploads in tmp/uploads for the admin to delete.
• An invalid cron expression leaves the task unscheduled, with an error in the startup log.

Kept as a draft until scripts/sources.toml is bumped to a release containing rommapp/romm#4957. The generated snippets are untouched here and get regenerated with that bump. Against the romm PR head, gen_env_vars picks up all eight new variables. gen_scheduled_tasks currently fails against romm master for an unrelated reason: reap_streaming_sessions.py passes a call for enabled=, which resolve() doesn't handle. That has to be fixed before the regen.

trunk check is clean. mkdocs build --strict only reports the usual local cairosvg warning.

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