refactor: one definition of a followed channel and of a choice spelling

The explore purge rebuilt followed_channel's two EXISTS clauses inline, so what is polled and what may be deleted could drift; it now uses the predicate, which moved to sync/explore.py. The env validator and sysconfig each folded a choice value on their own; both call config.canonical_choice. The deep-backfill comment no longer calls recent backfill unconditional.

backend/app/routes/scheduler.py

@@ -22,7 +22,8 @@ from app.scheduler import (
     trigger_all,
     trigger_job,
 )
-from app.sync.runner import estimate_deep_backfill, followed_channel
+from app.sync.explore import followed_channel
+from app.sync.runner import estimate_deep_backfill
 from app.routes._common import not_found
 
 router = APIRouter(prefix="/api/admin/scheduler", tags=["admin"])