feat: add structured backend logging

Collector failures, service-API errors, WS connection lifecycle, and
config reload/discovery events were previously invisible outside the
browser (or, for a few config/discovery cases, logged with an
inconsistent ad-hoc console.error). Add a small scoped logger
(lib/logger.ts, level via LOG_LEVEL) and wire it through server.ts,
the scheduler, service collector, config loader/effective store, and
the WS server so operators can see failures via `docker logs`.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

lib/config/effective.ts

@@ -1,6 +1,9 @@
 import { configStore } from "./loader";
 import { discoverDockerWidgets } from "@/lib/discovery/traefik-labels";
 import type { Config, Widget } from "./schema";
+import { createLogger } from "@/lib/logger";
+
+const log = createLogger("discovery");
 
 const DISCOVERY_INTERVAL_MS = 60_000;
 
@@ -53,11 +56,15 @@ class EffectiveConfigStore {
     try {
       const next = await discoverDockerWidgets(configStore.get());
       if (!sameDiscoveredSet(this.discovered, next)) {
+        log.info("discovered widget set changed", {
+          previous: this.discovered.length,
+          current: next.length,
+        });
         this.discovered = next;
         this.emit();
       }
     } catch (err) {
-      console.error(`[discovery] ${(err as Error).message}`);
+      log.error("discovery scan failed", { error: (err as Error).message });
     }
   }