perf(realtime+data): implement perf-data-optimization and perf-realtime-scale

## perf-data-optimization
- Add @@index([name]) on User model (migration)
- Add WEATHER_HISTORY_LIMIT=90 constant, apply take/orderBy on weather history queries
- Replace deep includes with explicit select on all 6 list service queries
- Add unstable_cache layer with revalidateTag on all list service functions
- Add cache-tags.ts helpers (sessionTag, sessionsListTag, userStatsTag)
- Invalidate sessionsListTag in all create/delete Server Actions

## perf-realtime-scale
- Create src/lib/broadcast.ts: generic createBroadcaster factory with shared polling
  (one interval per active session, starts on first subscriber, stops on last)
- Migrate all 6 SSE routes to use createBroadcaster — removes per-connection setInterval
- Add broadcastToXxx() calls in all Server Actions after mutations for immediate push
- Add SESSIONS_PAGE_SIZE=20, pagination on sessions page with loadMoreSessions action
- Add "Charger plus" button with loading state and "X sur Y" counter in WorkshopTabs

## Tests
- Add 19 unit tests for broadcast.ts (polling lifecycle, userId filtering,
  formatEvent, error resilience, session isolation)

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

src/app/api/sessions/route.ts

@@ -1,7 +1,9 @@
 import { NextResponse } from 'next/server';
+import { revalidateTag } from 'next/cache';
 import { auth } from '@/lib/auth';
 import { prisma } from '@/services/database';
 import { shareSession } from '@/services/sessions';
+import { sessionsListTag } from '@/lib/cache-tags';
 
 export async function GET() {
   try {
@@ -63,6 +65,7 @@ export async function POST(request: Request) {
       console.error('Auto-share failed:', shareError);
     }
 
+    revalidateTag(sessionsListTag(session.user.id), 'default');
     return NextResponse.json(newSession, { status: 201 });
   } catch (error) {
     console.error('Error creating session:', error);