refactor(vibe): simplify durable session flow

frontend/src/services/vibeService.test.ts

@@ -11,7 +11,7 @@ describe('durable vibe service', () => {
 
     await vibeService.next('session', 3);
 
-    expect(post).toHaveBeenCalledWith('/v2/vibe/sessions/session/next', { expectedPlanVersion: 3 });
+    expect(post).toHaveBeenCalledWith('/v2/vibe/sessions/session/advance', { expectedPlanVersion: 3 });
   });
 
   it('uses an explicit idempotency key to advance a served but unplayable item', async () => {
@@ -21,7 +21,7 @@ describe('durable vibe service', () => {
       eventId: 'event', planVersionId: 'plan', ordinal: 4, trackId: 'track',
     });
 
-    expect(post).toHaveBeenCalledWith('/v2/vibe/sessions/session/next', {
+    expect(post).toHaveBeenCalledWith('/v2/vibe/sessions/session/advance', {
       expectedPlanVersion: 3,
       unplayable: { eventId: 'event', planVersionId: 'plan', ordinal: 4, trackId: 'track' },
     });

frontend/src/services/vibeService.ts

@@ -71,7 +71,7 @@ export const vibeService = {
   },
 
   async next(sessionId: string, expectedPlanVersion: number): Promise<DurableVibeSessionResponse> {
-    const res = await api.post<DurableVibeSessionResponse>(`/v2/vibe/sessions/${sessionId}/next`, {
+    const res = await api.post<DurableVibeSessionResponse>(`/v2/vibe/sessions/${sessionId}/advance`, {
       expectedPlanVersion,
     });
     return res.data;
@@ -82,7 +82,7 @@ export const vibeService = {
     expectedPlanVersion: number,
     unplayable: VibeUnplayableItemInput,
   ): Promise<DurableVibeSessionResponse> {
-    const res = await api.post<DurableVibeSessionResponse>(`/v2/vibe/sessions/${sessionId}/next`, {
+    const res = await api.post<DurableVibeSessionResponse>(`/v2/vibe/sessions/${sessionId}/advance`, {
       expectedPlanVersion,
       unplayable,
     });

frontend/src/services/vibeSession.ts

@@ -202,7 +202,7 @@ function isSessionTerminalError(error: unknown): boolean {
 export function vibeErrorMessage(error: unknown): string {
   if (!axios.isAxiosError(error)) return 'Could not refresh this Vibe. Please try again.';
   switch (error.response?.status) {
-    case 401: return 'Vibe needs a trusted local user identity. Set MUZICK_VIBE_USER_ID and try again.';
+    case 400: return 'Vibe needs a valid user identity.';
     case 404: return 'This Vibe session is no longer available.';
     case 409: return 'This Vibe session has already ended or was replaced.';
     default: return 'Could not refresh this Vibe. Please try again.';

frontend/src/types.ts

@@ -75,8 +75,8 @@ export interface HealthResponse {
   redis: 'ok' | 'error' | 'unknown';
 }
 
-// Active v2 recommendation session. The sessionId comes from
-// POST /api/v2/vibe/start and identifies the Redis-stored plan.
+// Active durable recommendation session. The sessionId comes from
+// POST /api/v2/vibe/sessions and identifies its persisted plan.
 export interface VibeSession {
   sessionId: string;
   seedTrackId: string | null;