feat: add /api/dashboard endpoint with hero match, stats, streak, nudges

Returns next tippable match (hero), user rank/points from leaderboard,
consecutive-tip streak, and up to 3 contextual nudges in one request.
Mounts at /api/dashboard; adds getDashboard() + DashboardData type to
the frontend API client.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/index.ts

@@ -14,6 +14,7 @@ import leaderboardRouter from './routes/leaderboard';
 import adminRouter from './routes/admin';
 import profileRouter from './routes/profile';
 import devRouter from './routes/dev';
+import dashboardRouter from './routes/dashboard';
 
 const app = express();
 const PORT = parseInt(process.env.PORT ?? '3001');
@@ -126,6 +127,7 @@ app.use('/api/tips', tipsRouter);
 app.use('/api/leaderboard', leaderboardRouter);
 app.use('/api/admin', adminRouter);
 app.use('/api/profile', profileRouter);
+app.use('/api/dashboard', dashboardRouter);
 if (process.env.NODE_ENV === 'development') {
   app.use('/api/dev', devRouter);
 }