feat(profile): keep_screen_awake user preference (default on)

New users column surfaced as an On/Off dropdown in the profile form.
The form and API carry "true"/"false" strings with the theme-style
""-guard, so JSON clients that omit the field can't clobber it;
anything else is rejected. GET/PUT /api/auth/profile expose
keep_screen_awake alongside the other profile fields, and the reader
route threads the preference into its template metadata.

Integration tests pin the schema default, both values, omission
semantics, and rejection — dedicated test user, never testuser.

database/schema/schema.sql

@@ -42,6 +42,11 @@ CREATE TABLE IF NOT EXISTS users (
 -- mirrors the per-book table's column.
 ALTER TABLE users ADD COLUMN IF NOT EXISTS default_audio_speed REAL NOT NULL DEFAULT 1.0;
 
+-- Keep-screen-awake preference: when true, the web reader and audiobook
+-- player request a Screen Wake Lock so the display doesn't blank/lock
+-- mid-session. On by default; the profile page's On/Off dropdown owns it.
+ALTER TABLE users ADD COLUMN IF NOT EXISTS keep_screen_awake BOOLEAN NOT NULL DEFAULT TRUE;
+
 -- Create system_settings table
 CREATE TABLE IF NOT EXISTS system_settings (
     id UUID PRIMARY KEY DEFAULT gen_random_uuid(),