feat(db): archive lifecycle schema and queries for missing media

Add the storage behind the archive-instead-of-delete lifecycle:

- media_items.missing_scan_count (INT, default 0) and archived_at
  (TIMESTAMPTZ, partial index), both as idempotent ADD COLUMN IF NOT
  EXISTS backfills for existing installs.
- MarkMediaItemMissing / ArchiveMediaItem / ClearMediaItemArchive plus
  PurgeExpiredArchivedMediaItems (retention cutoff) and
  PurgeAllArchivedMediaItems (manual bulk), with CountArchivedMediaItems
  for the admin UI.
- Archived items are hidden from every user-facing listing:
  ListMediaItems, ListMediaItemsByLibrary, ListMediaItemsSorted,
  SearchMediaItems, SearchMediaItemsUnified, the next_books CTE, the
  library media counts, and the search autocomplete value lists. Detail
  lookups by id/path/sha are intentionally unfiltered, and a dedicated
  ListMediaItemsByLibraryIncludingArchived feeds the scanner so the
  lifecycle pass can see and restore archived rows.

internal/database/models.go

@@ -305,16 +305,18 @@ type MediaItems struct {
 	// Scan information from ComicInfo.xml (scanner group, resolution, etc.)
 	ScanInformation pgtype.Text `db:"scan_information" json:"scan_information"`
 	// Summary from ComicInfo.xml (may be merged with description from Calibre)
-	Summary            pgtype.Text `db:"summary" json:"summary"`
-	MetadataOverrides  []string    `db:"metadata_overrides" json:"metadata_overrides"`
-	ChapterMetadata    []byte      `db:"chapter_metadata" json:"chapter_metadata"`
-	LibraryTypeName    pgtype.Text `db:"library_type_name" json:"library_type_name"`
-	TagsSearch         []string    `db:"tags_search" json:"tags_search"`
-	ContributorsSearch []string    `db:"contributors_search" json:"contributors_search"`
-	FileSha256         pgtype.Text `db:"file_sha256" json:"file_sha256"`
-	OpfIdentifier      pgtype.Text `db:"opf_identifier" json:"opf_identifier"`
-	OpfUuid            pgtype.Text `db:"opf_uuid" json:"opf_uuid"`
-	HashConfidence     pgtype.Text `db:"hash_confidence" json:"hash_confidence"`
+	Summary            pgtype.Text        `db:"summary" json:"summary"`
+	MetadataOverrides  []string           `db:"metadata_overrides" json:"metadata_overrides"`
+	MissingScanCount   int32              `db:"missing_scan_count" json:"missing_scan_count"`
+	ArchivedAt         pgtype.Timestamptz `db:"archived_at" json:"archived_at"`
+	ChapterMetadata    []byte             `db:"chapter_metadata" json:"chapter_metadata"`
+	LibraryTypeName    pgtype.Text        `db:"library_type_name" json:"library_type_name"`
+	TagsSearch         []string           `db:"tags_search" json:"tags_search"`
+	ContributorsSearch []string           `db:"contributors_search" json:"contributors_search"`
+	FileSha256         pgtype.Text        `db:"file_sha256" json:"file_sha256"`
+	OpfIdentifier      pgtype.Text        `db:"opf_identifier" json:"opf_identifier"`
+	OpfUuid            pgtype.Text        `db:"opf_uuid" json:"opf_uuid"`
+	HashConfidence     pgtype.Text        `db:"hash_confidence" json:"hash_confidence"`
 }
 
 type MediaNotes struct {