feat(demo): Grafana + OTel Collector docker-compose demo

## Summary

Ship a docker-compose demo that lets users see UserALE behavioral data in Grafana in under 5 minutes.

## Components

```yaml
# example/otel-demo/docker-compose.yml
services:
collector: # OTel Collector with OTLP receiver
grafana: # Grafana with pre-provisioned dashboard
loki: # Log storage backend
example-app: # Example page with UserALE + OTLP exporter
```

## Pre-built Grafana Dashboard

- Event timeline (clicks, scrolls, keystrokes over time)
- Session heatmap (events by session)
- Top interaction targets (most-clicked elements)
- User flow (page URL transitions)
- Event type distribution

## First-run Experience

```bash
cd example/otel-demo
docker-compose up
open http://localhost:8080 # Example app
open http://localhost:3000 # Grafana dashboard
```

## Depends On

- #140 (OTLP exporter)

Start by reviewing example/otel-demo/docker-compose.yml and dependency #140, then verify how the collector, Grafana, Loki, and example app should connect. Run the documented docker-compose up flow and check both localhost:8080 and localhost:3000. Done means the demo starts within the stated flow and exposes the listed behavioral views in Grafana.

Written by the indexing model from the issue text.