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_live_api.py

@@ -90,7 +90,7 @@ class TestConfig:
 # ---------------------------------------------------------------------------
 
 EXPECTED_CONTAINERS = [
-    'cell-caddy', 'cell-dns', 'cell-dhcp', 'cell-ntp',
+    'cell-caddy', 'cell-dns', 'cell-ntp',
     'cell-mail', 'cell-radicale', 'cell-webdav', 'cell-wireguard',
     'cell-api', 'cell-webui', 'cell-rainloop', 'cell-filegator',
 ]
@@ -164,7 +164,7 @@ class TestWireGuard:
 
 
 # ---------------------------------------------------------------------------
-# Network services: DNS, DHCP, NTP
+# Network services: DNS, NTP
 # ---------------------------------------------------------------------------
 
 class TestNetworkServices:
@@ -176,8 +176,8 @@ class TestNetworkServices:
         r = get('/api/dns/status')
         assert r.status_code == 200
 
-    def test_dhcp_leases_endpoint(self):
-        r = get('/api/dhcp/leases')
+    def test_dns_overview_endpoint(self):
+        r = get('/api/dns/overview')
         assert r.status_code == 200
 
     def test_ntp_status_endpoint(self):