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.

internal/database/querier.go

@@ -699,6 +699,7 @@ type Querier interface {
 	// Update universal progress
 	UpdateUniversalProgress(ctx context.Context, arg UpdateUniversalProgressParams) (ReadingProgress, error)
 	UpdateUserDefaultAudioSpeed(ctx context.Context, arg UpdateUserDefaultAudioSpeedParams) error
+	UpdateUserKeepScreenAwake(ctx context.Context, arg UpdateUserKeepScreenAwakeParams) error
 	UpdateUserMaxDevices(ctx context.Context, arg UpdateUserMaxDevicesParams) (Users, error)
 	UpdateUserProfile(ctx context.Context, arg UpdateUserProfileParams) error
 	UpdateUserRole(ctx context.Context, arg UpdateUserRoleParams) (UpdateUserRoleRow, error)