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.

database/schema/schema.sql

@@ -213,6 +213,13 @@ CREATE TABLE IF NOT EXISTS media_items (
     -- reset-to-scanned-defaults action clears them.
     metadata_overrides TEXT[] NOT NULL DEFAULT '{}',
 
+    -- Archive lifecycle: a file missing from disk for two consecutive scans
+    -- gets archived_at set (hidden from all browsing, reading history kept).
+    -- If the file returns, the row is un-archived. Archived rows are purged
+    -- after ARCHIVE_RETENTION_DAYS (0 = manual purge only).
+    missing_scan_count INT NOT NULL DEFAULT 0,
+    archived_at TIMESTAMPTZ,
+
     -- Chapter metadata for reader navigation and progress tracking
         -- Caches detected chapter structure to avoid re-parsing files
         -- Populated by ReaderService.DetectChapters() on first read
@@ -243,6 +250,11 @@ ALTER TABLE media_items ADD COLUMN IF NOT EXISTS contributors_search TEXT[];
 -- definition above for fresh databases)
 ALTER TABLE media_items ADD COLUMN IF NOT EXISTS metadata_overrides TEXT[] NOT NULL DEFAULT '{}';
 
+-- Archive lifecycle (idempotent backfill for existing installs)
+ALTER TABLE media_items ADD COLUMN IF NOT EXISTS missing_scan_count INT NOT NULL DEFAULT 0;
+ALTER TABLE media_items ADD COLUMN IF NOT EXISTS archived_at TIMESTAMPTZ;
+CREATE INDEX IF NOT EXISTS idx_media_items_archived ON media_items (archived_at) WHERE archived_at IS NOT NULL;
+
 -- Create GIN indexes for fast search field searches
 CREATE INDEX IF NOT EXISTS idx_media_items_tags_search ON media_items USING GIN (tags_search);
 CREATE INDEX IF NOT EXISTS idx_media_items_contributors_search ON media_items USING GIN (contributors_search);