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/db.py

@@ -442,12 +442,18 @@ def init_db():
                 id INTEGER PRIMARY KEY AUTOINCREMENT,
                 page_id INTEGER NOT NULL REFERENCES pages(id) ON DELETE CASCADE,
                 shared_with_user_id INTEGER REFERENCES users(id),
+                shared_with_group_id INTEGER REFERENCES user_groups(id) ON DELETE CASCADE,
                 shared_with_email TEXT DEFAULT '',
                 permission TEXT NOT NULL DEFAULT 'view',
                 created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
                 created_by INTEGER REFERENCES users(id)
             )
         """)
+        # v5.x: migration — partage par groupes (colonne manquante sur DB existantes)
+        try:
+            conn.execute("ALTER TABLE page_shares ADD COLUMN shared_with_group_id INTEGER REFERENCES user_groups(id) ON DELETE CASCADE")
+        except sqlite3.OperationalError:
+            pass
         # 4) recents table
         conn.execute("""
             CREATE TABLE IF NOT EXISTS recents (