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/integration/test_negative_scenarios.py

@@ -11,7 +11,6 @@ Endpoints covered:
   - /api/peers        (POST, PUT, DELETE)
   - /api/config       (PUT)
   - /api/dns/records  (DELETE)
-  - /api/dhcp/reservations (POST, DELETE)
   - /api/containers/<name>/restart
   - /api/wireguard/keys/peer
 
@@ -240,43 +239,6 @@ class TestDnsRecordsNegative:
         r.json()
 
 
-# ---------------------------------------------------------------------------
-# DHCP reservations — negative
-# ---------------------------------------------------------------------------
-
-class TestDhcpReservationsNegative:
-    def test_add_reservation_no_body_returns_400(self):
-        r = _S.post(
-            f"{API_BASE}/api/dhcp/reservations",
-            data='',
-            headers={'Content-Type': 'application/json'},
-        )
-        assert r.status_code == 400
-
-    def test_add_reservation_missing_ip_returns_400(self):
-        r = post('/api/dhcp/reservations', json={'mac': 'aa:bb:cc:dd:ee:ff'})
-        assert r.status_code == 400
-        _assert_json_error(r)
-
-    def test_add_reservation_missing_mac_returns_400(self):
-        r = post('/api/dhcp/reservations', json={'ip': '10.0.0.250'})
-        assert r.status_code == 400
-        _assert_json_error(r)
-
-    def test_delete_reservation_no_mac_returns_400(self):
-        r = delete('/api/dhcp/reservations', json={'ip': '10.0.0.250'})
-        assert r.status_code == 400
-        _assert_json_error(r)
-
-    def test_delete_reservation_empty_body_returns_400(self):
-        r = _S.delete(
-            f"{API_BASE}/api/dhcp/reservations",
-            data='',
-            headers={'Content-Type': 'application/json'},
-        )
-        assert r.status_code == 400
-
-
 # ---------------------------------------------------------------------------
 # Container endpoints — negative
 # ---------------------------------------------------------------------------