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/conftest.py

@@ -26,7 +26,7 @@ def tmp_dir():
 @pytest.fixture
 def tmp_config_dir(tmp_dir):
     """Temporary config dir with the sub-directories expected by managers."""
-    for sub in ('api', 'caddy', 'dns', 'dhcp', 'ntp', 'wireguard'):
+    for sub in ('api', 'caddy', 'dns', 'ntp', 'wireguard'):
         os.makedirs(os.path.join(tmp_dir, sub), exist_ok=True)
     return tmp_dir