fix(media-items): normalize NOT NULL narrators on every write path

A2 made narrators/narrators_search NOT NULL DEFAULT '{}' (86f0af6), but an
explicit NULL in an INSERT/UPDATE column list bypasses the column default
— the API media-create path (POST /api/media-items without narrators)
500'd on every call, taking the whole integration suite down with it
(ISBN tests, capture seeds, all direct-DB seeds).

- set_library_type_name() trigger now coalesces NULL narrators/
  narrators_search to '{}' at the row level, and its update trigger
  widens from BEFORE UPDATE OF library_id to BEFORE UPDATE so every
  write path is covered
- defense in depth at the callers: handler create/update normalize nil
  to empty slices (normalizeNarrators), scanner seeds the same
  (nilToEmpty) for files that carry no narrator credits
- analytics device-usage tests move to the dedicated per-test user
  (the shared admin's device list accrues live rows mid-suite — the
  documented testuser flapping class)

internal/services/media_scanner.go

@@ -169,6 +169,17 @@ type CalibreOPFMetadata struct {
 // watcher (and thus can never panic on EMFILE/ENOSPC). This fixes the
 // fd/inotify-watch leak where every scan job created a watcher that was
 // never closed.
+// nilToEmpty guards the NOT NULL narrators columns (A2): a Go nil slice
+// becomes an explicit NULL in INSERT/UPDATE params and 23502s — the
+// column DEFAULT never fires for explicit NULLs. "No narrators known"
+// is an empty array, never NULL.
+func nilToEmpty(slice []string) []string {
+	if slice == nil {
+		return []string{}
+	}
+	return slice
+}
+
 func NewMediaScanner(db *database.Queries) *MediaScanner {
 	return &MediaScanner{
 		db:                   db,
@@ -947,7 +958,7 @@ func (s *MediaScanner) processMediaFile(ctx context.Context, path string) (bool,
 		DatePublished:      pgtype.Date{Time: metadata.PublishDate, Valid: !metadata.PublishDate.IsZero()},
 		Contributors:       metadata.Contributors,
 		ContributorsSearch: contributorsSearch,
-		Narrators:          metadata.Narrators,
+		Narrators:          nilToEmpty(metadata.Narrators),
 		NarratorsSearch:    utils.NormalizeNarratorsSearch(metadata.Narrators),
 		Abridged:           metadata.Abridged,
 		Tags:               metadata.Tags,
@@ -3210,7 +3221,7 @@ func (s *MediaScanner) updateMediaItem(ctx context.Context, existing database.Me
 		CoverImagePath:     pgtype.Text{String: s.getRelativePath(metadata.CoverPath), Valid: metadata.CoverPath != ""},
 		Series:             pgtype.Text{String: metadata.Series, Valid: metadata.Series != ""},
 		SeriesNumber:       pgtype.Int4{Int32: metadata.SeriesNumber, Valid: metadata.SeriesNumber > 0},
-		Narrators:          metadata.Narrators,
+		Narrators:          nilToEmpty(metadata.Narrators),
 		NarratorsSearch:    utils.NormalizeNarratorsSearch(metadata.Narrators),
 		Abridged:           metadata.Abridged,
 		Tags:               metadata.Tags,