fix: default seed_at to 00:00 (HH:MM)

Keep daily tasks next_run=0 startup catch-up; only move midnight
seed schedule default from 00:01 to 00:00 across config/docs/tests.

config.example.json

@@ -74,7 +74,7 @@
   "schedule": {
     "discover_minutes": 60,
     "snapshot_at": "23:15",
-    "seed_at": "00:01",
+    "seed_at": "00:00",
     "candidate_ttl_days": 7,
     "recent_snapshot_minutes": 60,
     "recent_video_days": 2