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/weekly-checkin.ts

@@ -1,8 +1,10 @@
 'use server';
 
-import { revalidatePath } from 'next/cache';
+import { revalidatePath, revalidateTag } from 'next/cache';
 import { auth } from '@/lib/auth';
 import * as weeklyCheckInService from '@/services/weekly-checkin';
+import { sessionsListTag } from '@/lib/cache-tags';
+import { broadcastToWeeklyCheckInSession } from '@/app/api/weekly-checkin/[id]/subscribe/route';
 import type { WeeklyCheckInCategory, Emotion } from '@prisma/client';
 
 // ============================================
@@ -36,6 +38,7 @@ export async function createWeeklyCheckInSession(data: {
     }
     revalidatePath('/weekly-checkin');
     revalidatePath('/sessions');
+    revalidateTag(sessionsListTag(session.user.id), 'default');
     return { success: true, data: weeklyCheckInSession };
   } catch (error) {
     console.error('Error creating weekly check-in session:', error);
@@ -63,9 +66,11 @@ export async function updateWeeklyCheckInSession(
       data
     );
 
+    broadcastToWeeklyCheckInSession(sessionId, { type: 'SESSION_UPDATED' });
     revalidatePath(`/weekly-checkin/${sessionId}`);
     revalidatePath('/weekly-checkin');
     revalidatePath('/sessions');
+    revalidateTag(sessionsListTag(authSession.user.id), 'default');
     return { success: true };
   } catch (error) {
     console.error('Error updating weekly check-in session:', error);
@@ -83,6 +88,7 @@ export async function deleteWeeklyCheckInSession(sessionId: string) {
     await weeklyCheckInService.deleteWeeklyCheckInSession(sessionId, authSession.user.id);
     revalidatePath('/weekly-checkin');
     revalidatePath('/sessions');
+    revalidateTag(sessionsListTag(authSession.user.id), 'default');
     return { success: true };
   } catch (error) {
     console.error('Error deleting weekly check-in session:', error);
@@ -128,6 +134,7 @@ export async function createWeeklyCheckInItem(
       }
     );
 
+    broadcastToWeeklyCheckInSession(sessionId, { type: 'ITEM_CREATED' });
     revalidatePath(`/weekly-checkin/${sessionId}`);
     return { success: true, data: item };
   } catch (error) {
@@ -169,6 +176,7 @@ export async function updateWeeklyCheckInItem(
       }
     );
 
+    broadcastToWeeklyCheckInSession(sessionId, { type: 'ITEM_UPDATED' });
     revalidatePath(`/weekly-checkin/${sessionId}`);
     return { success: true, data: item };
   } catch (error) {
@@ -203,6 +211,7 @@ export async function deleteWeeklyCheckInItem(itemId: string, sessionId: string)
       { itemId }
     );
 
+    broadcastToWeeklyCheckInSession(sessionId, { type: 'ITEM_DELETED' });
     revalidatePath(`/weekly-checkin/${sessionId}`);
     return { success: true };
   } catch (error) {
@@ -246,6 +255,7 @@ export async function moveWeeklyCheckInItem(
       }
     );
 
+    broadcastToWeeklyCheckInSession(sessionId, { type: 'ITEM_MOVED' });
     revalidatePath(`/weekly-checkin/${sessionId}`);
     return { success: true };
   } catch (error) {
@@ -284,6 +294,7 @@ export async function reorderWeeklyCheckInItems(
       { category, itemIds }
     );
 
+    broadcastToWeeklyCheckInSession(sessionId, { type: 'ITEMS_REORDERED' });
     revalidatePath(`/weekly-checkin/${sessionId}`);
     return { success: true };
   } catch (error) {