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/handlers/commonhandlers.go

@@ -65,6 +65,17 @@ func parseDate(dateStr string) time.Time {
 	return time.Time{}
 }
 
+// normalizeNarrators 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. Omitted narrators mean
+// "none known", which is an empty array, never NULL.
+func normalizeNarrators(narrators []string) []string {
+	if narrators == nil {
+		return []string{}
+	}
+	return narrators
+}
+
 func SetupRoutes(g *echo.Group, db *database.Queries, connManager *wsync.ConnectionManager, queueProcessor *wsync.SyncQueueProcessor, cfg *config.Config) *Handler {
 	return NewHandler(db, connManager, queueProcessor, cfg)
 }

internal/handlers/media.go

@@ -1414,7 +1414,8 @@ func (mh *MediaHandler) CreateMediaItem(c *echo.Context) error {
 
 	tagsSearch := utils.NormalizeTagsSearch(req.Tags)
 	contributorsSearch := utils.NormalizeContributorsSearch(req.Contributors)
-	narratorsSearch := utils.NormalizeNarratorsSearch(req.Narrators)
+	narrators := normalizeNarrators(req.Narrators)
+	narratorsSearch := utils.NormalizeNarratorsSearch(narrators)
 
 	// Validate and normalize ISBN
 	normalizedISBN, err := utils.NormalizeISBN(req.ISBN)
@@ -1464,7 +1465,7 @@ func (mh *MediaHandler) CreateMediaItem(c *echo.Context) error {
 		SeriesNumber:       pgtype.Int4{Int32: req.SeriesNumber, Valid: req.SeriesNumber > 0},
 		Tags:               req.Tags,
 		TagsSearch:         tagsSearch,
-		Narrators:          req.Narrators,
+		Narrators:          narrators,
 		NarratorsSearch:    narratorsSearch,
 		Abridged:           req.Abridged,
 		Asin:               pgtype.Text{String: req.ASIN, Valid: req.ASIN != ""},
@@ -1532,7 +1533,8 @@ func (mh *MediaHandler) UpdateMediaItem(c *echo.Context) error {
 
 	tagsSearch := utils.NormalizeTagsSearch(req.Tags)
 	contributorsSearch := utils.NormalizeContributorsSearch(req.Contributors)
-	narratorsSearch := utils.NormalizeNarratorsSearch(req.Narrators)
+	narrators := normalizeNarrators(req.Narrators)
+	narratorsSearch := utils.NormalizeNarratorsSearch(narrators)
 
 	normalizedISBN, err := utils.NormalizeISBN(req.ISBN)
 	if err != nil && req.ISBN != "" {
@@ -1586,7 +1588,7 @@ func (mh *MediaHandler) UpdateMediaItem(c *echo.Context) error {
 		SeriesNumber:       pgtype.Int4{Int32: req.SeriesNumber, Valid: req.SeriesNumber > 0},
 		Tags:               req.Tags,
 		TagsSearch:         tagsSearch,
-		Narrators:          req.Narrators,
+		Narrators:          narrators,
 		NarratorsSearch:    narratorsSearch,
 		Abridged:           req.Abridged,
 		Asin:               pgtype.Text{String: req.ASIN, Valid: req.ASIN != ""},