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>

compose.yaml

@@ -11,7 +11,13 @@ services:
     environment:
       PORT: "3000"
       CHOKIDAR_USEPOLLING: "true"
+      LOG_LEVEL: "${LOG_LEVEL:-info}"
     init: true
+    logging:
+      driver: json-file
+      options:
+        max-size: "20m"
+        max-file: "10"
     ports:
       - "3000:3000"
     volumes:
@@ -45,7 +51,13 @@ services:
       API_INTERNAL_URL: http://api:3000
       WATCHPACK_POLLING: "true"
       NEXT_TELEMETRY_DISABLED: "1"
+      LOG_LEVEL: "${LOG_LEVEL:-info}"
     init: true
+    logging:
+      driver: json-file
+      options:
+        max-size: "20m"
+        max-file: "10"
     depends_on:
       api:
         condition: service_healthy