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/routers/board.py

@@ -612,12 +612,21 @@ def _load_shared_sidebar_pages(user_id: int) -> tuple[list, list, list, list]:
             f"ORDER BY updated_at DESC LIMIT 20",
             scope_params,
         ).fetchall()
-        received_rows = conn.execute(
-            "SELECT DISTINCT p.id, p.title, p.workspace, p.updated_at FROM page_shares s "
-            "JOIN pages p ON p.id=s.page_id "
-            "WHERE s.shared_with_user_id=? AND p.deleted_at IS NULL",
-            (user_id,),
-        ).fetchall()
+        try:
+            received_rows = conn.execute(
+                "SELECT DISTINCT p.id, p.title, p.workspace, p.updated_at FROM page_shares s "
+                "JOIN pages p ON p.id=s.page_id "
+                "LEFT JOIN group_members gm ON gm.group_id = s.shared_with_group_id AND gm.user_id=? "
+                "WHERE (s.shared_with_user_id=? OR gm.user_id=?) AND p.deleted_at IS NULL",
+                (user_id, user_id, user_id),
+            ).fetchall()
+        except Exception:
+            received_rows = conn.execute(
+                "SELECT DISTINCT p.id, p.title, p.workspace, p.updated_at FROM page_shares s "
+                "JOIN pages p ON p.id=s.page_id "
+                "WHERE s.shared_with_user_id=? AND p.deleted_at IS NULL",
+                (user_id,),
+            ).fetchall()
 
     def _entry(r, icon):
         return {