feat(obs): Prometheus metrics on /metrics + golden-signal Grafana dashboard (closes #124) (#129)

## What & why

S-16c, the last of the S-16 (#17) split, on top of the backplane (#122) and distributed tracing (#123). The five .NET services now expose OpenTelemetry **metrics** in Prometheus format at `/metrics`; Prometheus scrapes each (one job per service); and Grafana ships a pre-built **Request path — golden signals** dashboard (traffic / errors / latency / saturation), split by service.

Closes #124

### How

- Each service adds `.WithMetrics(AddAspNetCoreInstrumentation + AddHttpClientInstrumentation + AddMeter("System.Runtime") + AddPrometheusExporter)` and maps `/metrics`. Same shape as the S-16b tracing wiring already in these `Program.cs` files.
- `infra/observability/prometheus/prometheus.yml`: one scrape job per service (`acl`, `domain`, `bff`, `event-subscriber`, `projection-api`), reached by compose service name.
- `infra/observability/grafana/provisioning/dashboards/`: dashboard provider + `golden-signals.json` (baked into the Grafana image by the existing `COPY provisioning/`).
- `verify-metrics` (new CI verify-stack step + Makefile target): generates BFF traffic and asserts Prometheus scraped the golden-signal metric from every service. Mirrors `verify-tracing`.

### Dependency (CLAUDE.md §13/§14)

Adds `OpenTelemetry.Exporter.Prometheus.AspNetCore` `1.17.0-beta.1` (matched to the `1.17.0` core already in use). It gives the OTel-native `/metrics` pull endpoint; replacing it would mean hand-rolling Prometheus exposition over a `MeterListener`; the risk is that it is a **prerelease** package (the whole OTel .NET Prometheus line is `-beta`) — pinned, wired only in `Program.cs`, and gated by `verify-metrics`. Recorded in **ADR-0024**.

## Definition of Done

- [x] Linked Gitea issue (#124).
- [x] Failing test committed before the implementation (`test(bff): /metrics exposes http-server request duration`).
- [x] Implementation makes the test pass.
- [ ] CI green — pending Gitea Actions run.
- [x] `docker compose up` reaches green health within 3 min (backplane images unchanged in shape; not on the health gate, ADR-0023).
- [x] Docs updated — demo-script S-16c entry.
- [x] ADR added — ADR-0024.
- [x] Demo note in `docs/demo-script.md`.

## Notes for reviewers

- `/health` polls are counted as traffic (metrics aren't path-filtered, unlike traces). Fine for a demo dashboard and honest — real load stacks on top.
- `projection-api` has no Stryker config (unchanged); the four mutated services carry the metrics wiring in `Program.cs`, same as the merged S-16b tracing code.
- Metric names verified against a live service: `http_server_request_duration_seconds{,_bucket,_count}`, label `http_response_status_code`, `dotnet_process_cpu_time_seconds_total`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: #129

docs/demo-script.md

@@ -5,6 +5,34 @@ copy-pasteable walkthrough against a local `make up` stack.
 
 ---
 
+## S-16c — Prometheus metrics + golden-signal Grafana dashboard (#124, ADR-0023)
+
+**Outcome:** the five .NET services now expose OpenTelemetry metrics in Prometheus format at `/metrics`
+— ASP.NET Core + `HttpClient` instrumentation plus the built-in `System.Runtime` meter. Prometheus
+scrapes each service (one job per service), and a **pre-built Grafana dashboard** — *Request path —
+golden signals* — plots the four golden signals: **traffic** (req/s), **errors** (5xx/s), **latency**
+(p95 request duration), and **saturation** (CPU cores in use), split by service. It populates under load.
+
+```bash
+# 1. Automated (a CI verify-stack step): generate BFF traffic and assert Prometheus scraped the
+#    golden-signal metric from every service.
+make verify-metrics              # → OK — targets up: [...]; request metric scraped from: [...]
+
+# 2. By hand: drive the stack, generate some load, then open the dashboard.
+make up
+for i in $(seq 1 50); do curl -s localhost:8080/openbaar/register >/dev/null; done   # BFF → projection-api
+open http://localhost:3000       # Grafana → Dashboards → "Request path — golden signals"
+open http://localhost:9090/targets   # Prometheus → every service target UP
+```
+
+**The path:** each host adds `.WithMetrics(AddAspNetCoreInstrumentation + AddHttpClientInstrumentation +
+AddMeter("System.Runtime") + AddPrometheusExporter)` and maps `/metrics`; Prometheus scrapes
+`<service>:8080/metrics` (config in `infra/observability/prometheus/prometheus.yml`); Grafana ships the
+dashboard via provisioning against the fixed `prometheus` datasource uid. No metrics are pushed over
+OTLP — Prometheus pulls, so there is no collector hop (ADR-0023).
+
+---
+
 ## S-16b — distributed traces across the .NET services (#123, ADR-0023)
 
 **Outcome:** the five .NET services (BFF, Domain, ACL, projection-api, event-subscriber) now emit