Register 8 more keys in the sysconfig registry (quota daily budget + backfill
reserve, recent-backfill window, shorts-probe params, enrich/autotag batch sizes)
and route their reads through the DB-override-or-env resolver at every call site
(quota.py, sync/runner.py, sync/videos.py, sync/autotag.py, sync/maintenance.py,
routes/scheduler.py). All read sites already had a db session in scope. Reads are
read-through (no cache), matching app.state; admin can tune them live, no redeploy.
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