Add configurable structured logging

Adds a leveled JSON logger (error/warn/info/verbose/debug, controlled via
LOG_LEVEL) wired into the Express API (access log, error middleware,
crash handlers, memory heartbeat) and the Next.js server (page-request
proxy, instrumentation crash handlers, heartbeat). LOG_LEVEL is exposed
through compose.yaml, and both services now rotate their Docker logs
(json-file, 20m x 10 files) instead of growing unbounded. Intended to
capture long-running diagnostic data for the intermittent 502s seen on
the Docker host.

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

src/shared/logging/logger.ts

@@ -0,0 +1,71 @@
+export type LogLevel = "error" | "warn" | "info" | "verbose" | "debug";
+
+const LEVEL_SEVERITY: Record<LogLevel, number> = {
+  error: 0,
+  warn: 1,
+  info: 2,
+  verbose: 3,
+  debug: 4,
+};
+
+const DEFAULT_LEVEL: LogLevel = "info";
+
+export function resolveLogLevel(raw: string | undefined): LogLevel {
+  const candidate = raw?.trim().toLowerCase();
+  if (candidate && candidate in LEVEL_SEVERITY) {
+    return candidate as LogLevel;
+  }
+  return DEFAULT_LEVEL;
+}
+
+export function toErrorMeta(error: unknown): Record<string, unknown> {
+  if (error instanceof Error) {
+    return {
+      errorName: error.name,
+      errorMessage: error.message,
+      stack: error.stack,
+    };
+  }
+  return { error: typeof error === "string" ? error : JSON.stringify(error) };
+}
+
+export interface Logger {
+  error(message: string, meta?: Record<string, unknown>): void;
+  warn(message: string, meta?: Record<string, unknown>): void;
+  info(message: string, meta?: Record<string, unknown>): void;
+  verbose(message: string, meta?: Record<string, unknown>): void;
+  debug(message: string, meta?: Record<string, unknown>): void;
+}
+
+export function createLogger(
+  scope: string,
+  options?: { level?: LogLevel }
+): Logger {
+  const threshold = options?.level ?? resolveLogLevel(process.env.LOG_LEVEL);
+
+  const write = (level: LogLevel, message: string, meta?: Record<string, unknown>) => {
+    if (LEVEL_SEVERITY[level] > LEVEL_SEVERITY[threshold]) {
+      return;
+    }
+    const line = JSON.stringify({
+      timestamp: new Date().toISOString(),
+      level,
+      scope,
+      message,
+      ...meta,
+    });
+    if (level === "error" || level === "warn") {
+      console.error(line);
+    } else {
+      console.log(line);
+    }
+  };
+
+  return {
+    error: (message, meta) => write("error", message, meta),
+    warn: (message, meta) => write("warn", message, meta),
+    info: (message, meta) => write("info", message, meta),
+    verbose: (message, meta) => write("verbose", message, meta),
+    debug: (message, meta) => write("debug", message, meta),
+  };
+}