fix: derive HINDSIGHT_API_HEALTH_URL default from HINDSIGHT_API_PORT

[sg-shag]

Problem

HINDSIGHT_API_HEALTH_URL default is hardcoded to port 8888 and does not
respect HINDSIGHT_API_PORT. When a custom port is set, the health check
polls the wrong port and the container fails to start after 300s.

Fix

One-line change in docker/standalone/start-all.sh:

-    API_HEALTH_URL="${HINDSIGHT_API_HEALTH_URL:-http://localhost:8888/health}"
+    API_HEALTH_URL="${HINDSIGHT_API_HEALTH_URL:-http://localhost:${HINDSIGHT_API_PORT:-8888}/health}"

No behavior change when HINDSIGHT_API_PORT is not set — defaults to 8888 as before.

Closes #1708

[benfrank241]

@sg-shag thanks for the contribution!