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/library.py

@@ -235,15 +235,24 @@ async def library_shared(
     uid = _get_user_id(request)
 
     # Page ids the user shares toward others (nominal page_shares) or receives
+    # (direct shares + group shares via group_members)
     with get_conn() as conn:
         made_rows = conn.execute(
             "SELECT DISTINCT s.page_id FROM page_shares s WHERE s.created_by=?",
             (uid,),
         ).fetchall()
-        recv_rows = conn.execute(
-            "SELECT DISTINCT s.page_id FROM page_shares s WHERE s.shared_with_user_id=?",
-            (uid,),
-        ).fetchall()
+        try:
+            recv_rows = conn.execute(
+                """SELECT DISTINCT s.page_id FROM page_shares s
+                   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=?""",
+                (uid, uid, uid),
+            ).fetchall()
+        except Exception:
+            recv_rows = conn.execute(
+                "SELECT DISTINCT s.page_id FROM page_shares s WHERE s.shared_with_user_id=?",
+                (uid,),
+            ).fetchall()
     made_ids = {r[0] for r in made_rows}
     recv_ids = {r[0] for r in recv_rows}