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/actions/weather.ts

@@ -1,8 +1,9 @@
 'use server';
 
-import { revalidatePath } from 'next/cache';
+import { revalidatePath, revalidateTag } from 'next/cache';
 import { auth } from '@/lib/auth';
 import * as weatherService from '@/services/weather';
+import { sessionsListTag } from '@/lib/cache-tags';
 import { getUserById } from '@/services/auth';
 import { broadcastToWeatherSession } from '@/app/api/weather/[id]/subscribe/route';
 
@@ -20,6 +21,7 @@ export async function createWeatherSession(data: { title: string; date?: Date })
     const weatherSession = await weatherService.createWeatherSession(session.user.id, data);
     revalidatePath('/weather');
     revalidatePath('/sessions');
+    revalidateTag(sessionsListTag(session.user.id), 'default');
     return { success: true, data: weatherSession };
   } catch (error) {
     console.error('Error creating weather session:', error);
@@ -65,6 +67,7 @@ export async function updateWeatherSession(
     revalidatePath(`/weather/${sessionId}`);
     revalidatePath('/weather');
     revalidatePath('/sessions');
+    revalidateTag(sessionsListTag(authSession.user.id), 'default');
     return { success: true };
   } catch (error) {
     console.error('Error updating weather session:', error);
@@ -82,6 +85,7 @@ export async function deleteWeatherSession(sessionId: string) {
     await weatherService.deleteWeatherSession(sessionId, authSession.user.id);
     revalidatePath('/weather');
     revalidatePath('/sessions');
+    revalidateTag(sessionsListTag(authSession.user.id), 'default');
     return { success: true };
   } catch (error) {
     console.error('Error deleting weather session:', error);