Add Prometheus metrics endpoint and Grafana dashboard

[Copilot]

Adds observability to Gee Bee via a Prometheus metrics endpoint and a pre-built, auto-provisioned Grafana dashboard.

Metrics (internal/metrics/metrics.go)

Four counters registered via promauto and served at http://0.0.0.0:<port>/metrics:

Metric	Labels	Description
geebee_aircraft_spotted_total	tail_number	Aircraft spotted per registration
geebee_api_calls_total	—	Upstream ADS-B API calls
geebee_api_errors_total	—	Upstream ADS-B API errors
geebee_notifications_sent_total	channel	Notifications sent (discord, slack, custom_url, terminal)

Port defaults to 9090, configurable via METRICS_PORT env var or metricsPort in config.

Grafana Dashboard (grafana/provisioning/dashboards/gee-bee.json)

7-panel dashboard covering rate timeseries for aircraft spotted and notifications by channel, plus stat panels for all four counters. Datasource is templated so it works across Grafana instances.

Docker Compose

docker compose up brings up all three services — gee-bee (:9090), Prometheus (:9091), and Grafana (:3000) — with the dashboard and datasource auto-provisioned.

prometheus:
  image: prom/prometheus:latest
  volumes:
    - ./prometheus.yml:/etc/prometheus/prometheus.yml:ro

grafana:
  image: grafana/grafana:latest
  volumes:
    - ./grafana/provisioning:/etc/grafana/provisioning:ro