feat(server): retire scan scheduler for watcher + safety-net walk

Wire the fsnotify watcher and a fixed 12h safety-net delta walk in main;
remove the configurable scan scheduler (scheduler.go, scan_schedule table via
migration 0033, GET/PATCH /api/admin/scan/schedule, and the server/api
plumbing). Manual scan + scan status are unchanged.

internal/db/migrations/0033_drop_scan_schedule.up.sql

@@ -0,0 +1,4 @@
+-- Retire the configurable scan scheduler. New-file pickup is now handled by
+-- the fsnotify watcher + a hardcoded safety-net delta walk (cmd/minstrel),
+-- so the operator-configurable schedule (and its admin UI) is gone.
+DROP TABLE IF EXISTS scan_schedule;