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.

templates/types.go

@@ -20,6 +20,8 @@ type User struct {
 	// User-level default audiobook playback speed (plan §8); 0 means
 	// "unset" and renders as the 1× option.
 	DefaultAudioSpeed float64
+	// Whether the web reader/player should hold a Screen Wake Lock.
+	KeepScreenAwake bool
 }
 
 // SpeedOption is one entry of the default-playback-speed select — the
@@ -289,6 +291,9 @@ type ReaderMetadata struct {
 	// TreatAsComic: fixed-layout item in a comics/manga library gets
 	// comic reader treatment (bookmarks only — no highlights, no search).
 	TreatAsComic bool `json:"treat_as_comic"`
+	// KeepScreenAwake: user preference — the reader holds a Screen Wake
+	// Lock while open when true.
+	KeepScreenAwake bool `json:"keep_screen_awake"`
 }
 
 type ReadingProgress struct {