feat(scanner): archive-at-two-scans lifecycle for files missing from disk

Replace the silent hard-delete orphan cleanup (which logged only through
ScannerLogger file logs and whose failure paths left rows undetected)
with an archive lifecycle that preserves reading history:

- A file missing in one scan is marked (missing_scan_count = 1); missing
  in a second consecutive scan archives it (archived_at, hidden from
  browsing, progress/notes/highlights survive). Every branch logs to
  stdout with an [ARCHIVE] prefix so skips are always visible.
- When a file reappears - same path, or identical content at a new path
  via the SHA-256 dedup match - the archived state clears automatically
  and the item returns with its history intact.
- Archived rows older than ARCHIVE_RETENTION_DAYS are hard-purged at
  scan time (cascading deletes); 0 disables auto-purge for manual-only
  management. Retention is read from the environment in NewMediaScanner.

internal/config/config.go

@@ -84,3 +84,14 @@ func getEnvInt(key string, defaultValue int) int {
 	}
 	return defaultValue
 }
+
+// ArchiveRetentionDays returns how many days a media item stays archived
+// (file missing from disk for two consecutive scans) before library scans
+// purge it for good. Reading progress, notes, and highlights survive the
+// archive window and are restored if the file returns; the purge deletes
+// them along with the row.
+// Configure via ARCHIVE_RETENTION_DAYS (default 90); 0 keeps archived items
+// until an admin purges them manually from the library admin page.
+func ArchiveRetentionDays() int {
+	return getEnvInt("ARCHIVE_RETENTION_DAYS", 90)
+}