feat: port conflict validation, autosave on Apply, extended integration tests

Port conflict validation:
- api/port_registry.py: detect_conflicts() checks all service sections for shared port values
- api/app.py: returns HTTP 409 on port conflict after existing range validation
- webui/src/pages/Settings.jsx: JS-side detectPortConflicts() with useMemo shows inline
  conflict errors and blocks Save before the request is made; catch blocks surface server
  error messages (including 409) instead of generic fallbacks

Config autosave on Apply:
- webui/src/contexts/DraftConfigContext.jsx: new context; Settings registers flush callbacks
  per section; App calls flushAll() before applyPending() when any section is dirty
- webui/src/App.jsx: wraps tree with DraftConfigProvider, handleApply shows 'saving' banner
  state and awaits flushAll()
- webui/src/pages/Settings.jsx: registers identity + per-service flushers; propagates dirty
  state into context via setDirty; uses refs to avoid stale closures

Extended integration test coverage (114 new tests):
- tests/integration/test_config_api.py: GET/PUT config, export, import, backup lifecycle
- tests/integration/test_network_services.py: DNS records + DHCP reservations CRUD
- tests/integration/test_containers.py: list, restart, logs, stats; recovery polling
- tests/integration/test_negative_scenarios.py: error-path coverage for all endpoints
- tests/test_port_conflicts.py: 20 unit tests for port_registry.detect_conflicts()

Pre-commit hook updated to skip tests/integration/ (live-stack tests require a running
stack and must be run explicitly via `make test-integration`).

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

webui/src/contexts/DraftConfigContext.jsx

@@ -0,0 +1,37 @@
+import { createContext, useContext, useRef, useCallback } from 'react';
+
+const DraftConfigContext = createContext(null);
+
+export function DraftConfigProvider({ children }) {
+  const flushersRef = useRef({});  // key → async flush fn
+
+  const registerFlusher = useCallback((key, fn) => {
+    flushersRef.current[key] = fn;
+    return () => { delete flushersRef.current[key]; };  // cleanup
+  }, []);
+
+  const hasDirtyRef = useRef({});  // key → boolean
+
+  const setDirty = useCallback((key, isDirty) => {
+    hasDirtyRef.current[key] = isDirty;
+  }, []);
+
+  const hasDirty = useCallback(() => {
+    return Object.values(hasDirtyRef.current).some(Boolean);
+  }, []);
+
+  const flushAll = useCallback(async () => {
+    const flushers = Object.values(flushersRef.current);
+    await Promise.all(flushers.map(fn => fn()));
+  }, []);
+
+  return (
+    <DraftConfigContext.Provider value={{ registerFlusher, setDirty, hasDirty, flushAll }}>
+      {children}
+    </DraftConfigContext.Provider>
+  );
+}
+
+export function useDraftConfig() {
+  return useContext(DraftConfigContext);
+}