test: raise coverage 68.7% -> ~80.4%; add ~250 tests for new egress/DDNS/network paths

Coverage was below acceptable levels and several newly-added code paths
(sshuttle egress, proxy egress, DDNS provider stubs, DNS overview route,
peer-registry provisioning) had zero test coverage.

~250 new unit tests are added across 16 new test files. Existing test files
are updated to match refactored interfaces (DHCP removed, constants
introduced, network_manager restructured). .coveragerc is added to pin the
source mapping and the 70% floor so regressions are caught at commit time.

tests/test_enhanced_api.py was previously living in api/ (wrong location)
and is moved to tests/ where it belongs.

Integration test files are updated to remove references to DHCP endpoints
and add coverage for the new DNS overview and DDNS sync endpoints.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_ddns_endpoints.py

@@ -169,5 +169,29 @@ class TestDdnsRegister(unittest.TestCase):
         self.assertTrue(body['registered'])
         self.assertEqual(body['subdomain'], 'mypic.pic.ngo')
 
+
+class TestDdnsSyncRecords(unittest.TestCase):
+
+    def setUp(self):
+        self.client = _make_client()
+
+    def test_sync_success(self):
+        from app import ddns_manager
+        with patch.object(ddns_manager, 'sync_service_records',
+                          return_value={'success': True, 'synced': ['a'], 'failed': []}):
+            r = self.client.post('/api/ddns/sync')
+        self.assertEqual(r.status_code, 200)
+        self.assertTrue(json.loads(r.data)['success'])
+
+    def test_sync_ddns_error_returns_400(self):
+        from app import ddns_manager
+        from ddns_manager import DDNSError
+        with patch.object(ddns_manager, 'sync_service_records',
+                          side_effect=DDNSError('no provider')):
+            r = self.client.post('/api/ddns/sync')
+        self.assertEqual(r.status_code, 400)
+        self.assertIn('error', json.loads(r.data))
+
+
 if __name__ == '__main__':
     unittest.main()