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)

database/schema/schema.sql

@@ -289,12 +289,25 @@ DROP TRIGGER IF EXISTS set_library_type_name_on_library_change ON media_items;
 DROP FUNCTION IF EXISTS set_library_type_name();
 
 -- Add trigger to populate library_type_name on insert
+-- Recompute the denormalized library type on insert, and NORMALIZE the
+-- A2 NOT NULL narrators columns: narrators/narrators_search are NOT NULL
+-- DEFAULT '{}' but an explicit NULL in an INSERT/UPDATE column list
+-- bypasses the column default and 23502s (the API create path without
+-- narrators 500'd exactly this way). "No narrators known" is an empty
+-- array, never NULL — coalesce at the row level so every write path
+-- (API, scanner, direct seeds) is safe by construction.
 CREATE OR REPLACE FUNCTION set_library_type_name() RETURNS TRIGGER AS $$
 BEGIN
   SELECT lt.name INTO NEW.library_type_name
   FROM libraries l
   JOIN library_types lt ON l.library_type_id = lt.id
   WHERE l.id = NEW.library_id;
+  IF NEW.narrators IS NULL THEN
+    NEW.narrators := '{}';
+  END IF;
+  IF NEW.narrators_search IS NULL THEN
+    NEW.narrators_search := '{}';
+  END IF;
   RETURN NEW;
 END;
 $$ LANGUAGE plpgsql;
@@ -305,9 +318,10 @@ CREATE TRIGGER set_library_type_name_on_insert
 
 -- Keep library_type_name truthful when an item is repointed to another
 -- library (cross-library move detection reuses the same function; the row
--- keeps its ID while its library changes).
+-- keeps its ID while its library changes). Fires on ALL updates so the
+-- narrators normalization guards every write path, not just library moves.
 CREATE TRIGGER set_library_type_name_on_library_change
-  BEFORE UPDATE OF library_id ON media_items
+  BEFORE UPDATE ON media_items
   FOR EACH ROW
   EXECUTE FUNCTION set_library_type_name();