feat(audiobooks): migration 142 add audiobooks.enabled flag

Server-settings row that gates the absorbed audiobooks feature.
Defaults to 'false' so sub-plan 1 lands as a strict no-op; subsequent
sub-plans branch on this flag and operators flip it to 'true' at
cutover.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

migrations/142_audiobooks_feature_flag.down.sql

@@ -0,0 +1 @@
+DELETE FROM server_settings WHERE key = 'audiobooks.enabled';

migrations/142_audiobooks_feature_flag.up.sql

@@ -0,0 +1,6 @@
+-- Master kill-switch for the absorbed audiobooks feature. Defaults to
+-- 'false' so landing migrations 139-142 is a no-op for users; operators
+-- flip this to 'true' at cutover.
+
+INSERT INTO server_settings (key, value) VALUES ('audiobooks.enabled', 'false')
+ON CONFLICT (key) DO NOTHING;