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/logger.ts

@@ -0,0 +1,47 @@
+type LogLevel = "debug" | "info" | "warn" | "error";
+type LogFields = Record<string, unknown>;
+
+const LEVEL_ORDER: Record<LogLevel, number> = { debug: 0, info: 1, warn: 2, error: 3 };
+
+function resolveMinLevel(): LogLevel {
+  const configured = process.env.LOG_LEVEL?.toLowerCase();
+  if (configured === "debug" || configured === "info" || configured === "warn" || configured === "error") {
+    return configured;
+  }
+  return process.env.NODE_ENV === "production" ? "info" : "debug";
+}
+
+const MIN_LEVEL = resolveMinLevel();
+
+function formatFields(fields?: LogFields): string {
+  if (!fields) return "";
+  const parts = Object.entries(fields)
+    .filter(([, value]) => value !== undefined)
+    .map(([key, value]) => `${key}=${typeof value === "string" ? value : JSON.stringify(value)}`);
+  return parts.length > 0 ? ` ${parts.join(" ")}` : "";
+}
+
+function write(level: LogLevel, scope: string, message: string, fields?: LogFields): void {
+  if (LEVEL_ORDER[level] < LEVEL_ORDER[MIN_LEVEL]) return;
+  const line = `${new Date().toISOString()} ${level.toUpperCase().padEnd(5)} [${scope}] ${message}${formatFields(fields)}`;
+  if (level === "error") console.error(line);
+  else if (level === "warn") console.warn(line);
+  else console.log(line);
+}
+
+export interface Logger {
+  debug(message: string, fields?: LogFields): void;
+  info(message: string, fields?: LogFields): void;
+  warn(message: string, fields?: LogFields): void;
+  error(message: string, fields?: LogFields): void;
+}
+
+/** Scoped logger. `scope` is the bracketed tag, e.g. createLogger("scheduler") -> "[scheduler]". */
+export function createLogger(scope: string): Logger {
+  return {
+    debug: (message, fields) => write("debug", scope, message, fields),
+    info: (message, fields) => write("info", scope, message, fields),
+    warn: (message, fields) => write("warn", scope, message, fields),
+    error: (message, fields) => write("error", scope, message, fields),
+  };
+}