Add per-event-type notification metrics

GET /metrics exposes one unlabeled counter for every notification ever published, regardless of what kind it was. A single number can tell ops that traffic went up; it can't tell them which feature is generating that traffic -- which matters the moment more than one part of the product publishes through this same endpoint. app/metrics.py already defines a per-event-type counter for exactly this purpose, but nothing increments it, so capacity planning and anomaly detection have no signal to work with.

app/metrics.py already defines events_published_by_type_total, a counter labeled by event, but nothing in app/publisher.py::publish_event ever calls .labels(...).inc() on it, so it sits permanently at zero regardless of traffic.

1. pytest practice_tickets/tests/test_ticket_06_event_type_metrics.py -v
2. Observe: publish_event() is called three times with event="order_shipped", and the labeled counter still reads 0.0 instead of 3.0.

A single unlabeled counter can tell you traffic went up; it can't tell you which feature is generating that traffic. This is a small, mechanical change, but it's the kind of thing that's much cheaper to add before there's an incident that needed it than after.

Look at how events_published_total.inc() is already called in app/publisher.py::publish_event, and how events_delivered_total.labels(source=...).inc() is called in app/streaming.py -- events_published_by_type_total needs the same .labels(...) treatment, keyed by this call's event value.