fix: mount sessionsRouter at /api/sessions (was /api — causait 404 sur toutes les routes sessions)

backend/src/index.ts

@@ -38,7 +38,7 @@ app.get('/api/health', (_req, res) => {
 
 app.use('/api/auth',     authRouter)
 app.use('/api/weapons',  weaponsRouter)
-app.use('/api',         sessionsRouter)
+app.use('/api/sessions', sessionsRouter)
 app.use('/api',          seriesRouter)
 app.use('/api',          miscRouter)
 app.use('/api',          analyzeRouter)