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.

types/model/prescene.go

@@ -0,0 +1,21 @@
+package model
+
+import "database/sql"
+
+// ScnlogMeta — структура, що кладеться в external_sources.meta (compact JSON).
+type ScnlogMeta struct {
+	SizeMb   *float64  `json:"size_mb"`
+	Genre    *string   `json:"genre"`
+	Artist   *[]string `json:"artist"`
+	Tracks   *int      `json:"tracks"`
+	Duration *string   `json:"duration"`
+}
+
+// ScnlogSource — рядок вибірки останнього необробленого prescene-джерела.
+type ScnlogSource struct {
+	Id       int64          `db:"id"`
+	TypeId   int64          `db:"type_id"`
+	Title    string         `db:"title"`
+	Meta     sql.NullString `db:"meta"`
+	ExSource string         `db:"eXsource"`
+}