feat(share,permissions): partage de page par groupes (page_shares)

- app/db.py: ajout colonne shared_with_group_id (FK user_groups, migration
  backfill v5.x) dans page_shares
- app/routers/sharing.py: POST /api/pages/{id}/share accepte group_id
  (upsert, verif FK groupe), GET /shares expose kind/group_name, synchro
  bidirectionnelle avec page_permissions (mirror grant/revoke) pour que
  PermissionManager donne un acces effectif (view/comment/edit) aux membres
  du groupe; PUT/DELETE gardent le miroir a jour
- app/routers/board.py, library.py: received/made incluent les partages via
  groupes (JOIN group_members)
- app/templates/_page_editor_content.html, _page_editor_scripts.html:
  dialogue Share — invite groups (fetch /api/v2/groups, filtre deja partages),
  pickInviteGroup, shareInvite(group_id), rendu accessList avec avatar groupe
- tests/test_share_groups.py: 10 tests (CRUD groupe, kind, miroir ACL)

Chore: inclut evolutions v6.4.0 deja en working copy (webhooks prod,
migrations, sync, config/main) pour garder l'arbre coherent.

app/main.py

@@ -98,13 +98,22 @@ async def lifespan(_app: FastAPI):
     from app.services.reminders import reminder_scheduler
     reminder_task = asyncio.create_task(reminder_scheduler())
 
+    # ── Webhooks outbound (v6.4.0): retry failed deliveries ──
+    from app.services.webhook_outbound import webhook_retry_scheduler
+    webhook_task = None
+    if settings.webhook_retry_enabled:
+        webhook_task = asyncio.create_task(webhook_retry_scheduler())
+
     logger.info("FlowDeck v%s started on port %d", dashboard._get_app_version(), settings.app_port)
     try:
         yield
     finally:
-        for task in (scheduler_task, automation_task, backup_task, projects_task, trash_task, reminder_task):
+        _tasks = (scheduler_task, automation_task, backup_task, projects_task, trash_task, reminder_task)
+        if webhook_task is not None:
+            _tasks = _tasks + (webhook_task,)
+        for task in _tasks:
             task.cancel()
-        for task in (scheduler_task, automation_task, backup_task, projects_task, trash_task, reminder_task):
+        for task in _tasks:
             try:
                 await task
             except asyncio.CancelledError: