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/context.js

@@ -111,10 +111,12 @@ const ctx = {
   logError: null,
   safeErrorMessage: null,
   buildDomain: null,
+  buildServiceUrl: null,
   getServiceById: null,
   readConfig: null,
   saveConfig: null,
   addServiceToConfig: null,
+  resyncHealthChecker: null,
   validateURL: null,
 
   // Middleware