Feature: add /api/metrics endpoint

[PRHubert]

Add a new /api/metrics endpoint to the Flask diagnostic app that returns
basic runtime statistics. This will give developers a quick snapshot of
app performance without needing external monitoring tools. The endpoint
should be consistent with the existing /api/report and /api/health routes.

Acceptance criteria:

• A new /api/metrics route is added to app.py
• The endpoint returns total requests served and memory usage
• The route appears in the existing API documentation in README.md