Unify URL resolution, add health checker sync, and make modules optional

- Add url-resolver.js with single resolveServiceUrl() used by all 5 consumers
  (probes, health routes, health checker auto-config)
- Health checker now does full sync (add/update/remove) instead of add-only,
  and re-syncs automatically after every services.json mutation
- docker-maintenance and log-digest are now optional imports with try/catch,
  preventing container crashes when these files are absent
- Add null guards in routes/logs.js for graceful 503 responses

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

dashcaddy-api/routes/health.js

@@ -6,6 +6,7 @@ const { TIMEOUTS } = require('../constants');
 const { exists } = require('../fs-helpers');
 const { paginate, parsePaginationParams } = require('../pagination');
 const platformPaths = require('../platform-paths');
+const { resolveServiceUrl } = require('../url-resolver');
 
 module.exports = function(ctx) {
   const router = express.Router();
@@ -36,18 +37,7 @@ module.exports = function(ctx) {
         let checkType = 'http';
 
         // Determine URL to check
-        if (service.isExternal && service.externalUrl) {
-          url = service.externalUrl;
-        } else if (service.containerId || service.containerName) {
-          // Local container - check via localhost and port
-          const port = service.port || 80;
-          url = `http://localhost:${port}`;
-        } else if (service.url) {
-          url = service.url.startsWith('http') ? service.url : `https://${service.url}`;
-        } else if (service.id) {
-          // Try common URL pattern
-          url = `https://${ctx.buildDomain(service.id)}`;
-        }
+        url = resolveServiceUrl(serviceId, service, ctx.siteConfig, ctx.buildServiceUrl);
 
         if (!url) {
           health[serviceId] = { status: 'unknown', reason: 'No URL configured' };
@@ -157,14 +147,7 @@ module.exports = function(ctx) {
     }
 
     // Determine URL
-    let url = null;
-    if (service.isExternal && service.externalUrl) {
-      url = service.externalUrl;
-    } else if (service.url) {
-      url = service.url.startsWith('http') ? service.url : `https://${service.url}`;
-    } else {
-      url = `https://${ctx.buildDomain(serviceId)}`;
-    }
+    const url = resolveServiceUrl(serviceId, service, ctx.siteConfig, ctx.buildServiceUrl);
 
     // Check health
     const controller = new AbortController();