feat(cells): Phase 2 — exit-offer signaling between connected cells

Adds the ability for a cell to signal to a peer that it's willing to
route internet traffic on their behalf.  This is the signaling layer
for Phase 3 (per-peer routing via exit cell).

Changes:
- cell_links.json: exit_offered (bool) + remote_exit_offered (bool)
  fields with lazy migration (default false for existing records)
- _push_permissions_to_remote: includes exit_offered in the push body
- apply_remote_permissions: accepts exit_offered kwarg; stores it as
  remote_exit_offered on the matching cell link
- peer-sync receiver: passes exit_offered from body to apply_remote_permissions
- CellLinkManager.set_exit_offered(cell_name, offered): persists +
  triggers push so the remote learns of our offer immediately
- PUT /api/cells/<name>/exit-offer: REST endpoint to toggle the flag
- 12 new tests covering all new paths

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_cells_endpoints.py

@@ -570,7 +570,8 @@ class TestPeerSyncPermissionsEndpoint(unittest.TestCase):
             environ_base={'REMOTE_ADDR': '10.1.0.5'},
         )
         mock_clm.apply_remote_permissions.assert_called_once_with(
-            'officepubkey=', self._VALID_BODY['permissions']
+            'officepubkey=', self._VALID_BODY['permissions'],
+            exit_offered=False,
         )
 
     @patch('app.cell_link_manager')
@@ -685,5 +686,55 @@ class TestPeerSyncPermissionsEndpoint(unittest.TestCase):
         self.assertEqual(r.status_code, 404)
 
 
+class TestSetExitOffer(unittest.TestCase):
+    """PUT /api/cells/<cell_name>/exit-offer"""
+
+    def setUp(self):
+        app.config['TESTING'] = True
+        self.client = app.test_client()
+
+    @patch('app.cell_link_manager')
+    def test_set_exit_offer_true_calls_set_exit_offered(self, mock_clm):
+        mock_clm.set_exit_offered.return_value = {'cell_name': 'remote', 'exit_offered': True}
+        r = self.client.put(
+            '/api/cells/remote/exit-offer',
+            data=json.dumps({'exit_offered': True}),
+            content_type='application/json',
+        )
+        self.assertEqual(r.status_code, 200)
+        mock_clm.set_exit_offered.assert_called_once_with('remote', True)
+
+    @patch('app.cell_link_manager')
+    def test_set_exit_offer_false_calls_set_exit_offered(self, mock_clm):
+        mock_clm.set_exit_offered.return_value = {'cell_name': 'remote', 'exit_offered': False}
+        r = self.client.put(
+            '/api/cells/remote/exit-offer',
+            data=json.dumps({'exit_offered': False}),
+            content_type='application/json',
+        )
+        self.assertEqual(r.status_code, 200)
+        mock_clm.set_exit_offered.assert_called_once_with('remote', False)
+
+    @patch('app.cell_link_manager')
+    def test_set_exit_offer_missing_field_returns_400(self, mock_clm):
+        r = self.client.put(
+            '/api/cells/remote/exit-offer',
+            data=json.dumps({}),
+            content_type='application/json',
+        )
+        self.assertEqual(r.status_code, 400)
+        mock_clm.set_exit_offered.assert_not_called()
+
+    @patch('app.cell_link_manager')
+    def test_set_exit_offer_unknown_cell_returns_404(self, mock_clm):
+        mock_clm.set_exit_offered.side_effect = ValueError("Cell 'nobody' not found")
+        r = self.client.put(
+            '/api/cells/nobody/exit-offer',
+            data=json.dumps({'exit_offered': True}),
+            content_type='application/json',
+        )
+        self.assertEqual(r.status_code, 404)
+
+
 if __name__ == '__main__':
     unittest.main()