fix(auth): emit default_audio_speed unconditionally on the profile response

TestDocsMatchRoutes failed on users/get_profile.md: the example carries
default_audio_speed but the response struct tagged it omitempty, so the
gate's zero-value marshal never sees the key and reads the doc as
inventing it. The tag was semantically dead: the column is NOT NULL
DEFAULT 1.0 (schema.sql 44) and writes validate 0.25-4, so a stored 0
is out of domain and every real profile emits the field. Response-only
struct, no form-bind impact. The key was added to the doc example in
ef827a3a without a gate run - this is exactly the class of drift the
gate exists to catch; make test now passes again.

internal/handlers/auth.go

@@ -124,8 +124,10 @@ type UserProfile struct {
 	Role      string `json:"role"`
 	Theme     string `json:"theme,omitempty"`
 	// User-level default audiobook playback speed (plan §8); 0 omits.
-	DefaultAudioSpeed float64 `json:"default_audio_speed,omitempty"`
-	// Whether the web reader/player should hold a Screen Wake Lock.
+	// Response-only struct: omit nothing — default_audio_speed is NOT NULL
+	// DEFAULT 1.0 (writes validate 0.25–4), so a real profile always emits
+	// it; omitempty here would drop a contract field from the docs gate.
+	DefaultAudioSpeed float64 `json:"default_audio_speed"`	// Whether the web reader/player should hold a Screen Wake Lock.
 	KeepScreenAwake bool `json:"keep_screen_awake"`
 	// Resume rewind: seconds before the saved position the audio
 	// player starts (open-resume and every genuine unpause). 0 is a