feat(prescene): parse single scnlog release for --single-uri page::1m

Handle the page::1m marker before the generic single-uri branch
(previously unreachable):

- select the latest prescene source from external_sources with meta IS NULL
  (sql.ErrNoRows -> no-op)
- parse article.single: type_id from post-(\d+), size_mb, genre,
  unique artists, tracks count and total duration (h:M:s)
- download the release NFO (anchor "Download NFO" -> onclick href)
  and save it to <nfo_dir>/<first-letter-of-title>/<title>.nfo
- update external_sources.meta with compact JSON by id via db.Exec,
  writing {} when every field is empty

Add model.ScnlogMeta / model.ScnlogSource structures.

config/scope/prescene.yaml

@@ -3,6 +3,7 @@ endpoint: https://scnlog.me/music/flac/
 endpoint-next: page/%s/
 sleep-before-next-iteration: 5s
 levels-to-scrape: 6
+nfo_dir: k:\data\mdb\nfo
 groups:
   tags:
     neformat: