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

services/bff/Bff.Api/Program.cs

@@ -3,6 +3,7 @@ using System.Text.Json;
 using System.Text.Json.Serialization;
 using Bff.Api;
 using Microsoft.AspNetCore.Authentication.JwtBearer;
+using OpenTelemetry.Metrics;
 using OpenTelemetry.Resources;
 using OpenTelemetry.Trace;
 
@@ -18,7 +19,16 @@ builder.Services.AddOpenTelemetry()
     .WithTracing(tracing => tracing
         .AddAspNetCoreInstrumentation(o => o.Filter = ctx => ctx.Request.Path != "/health")
         .AddHttpClientInstrumentation()
-        .AddOtlpExporter());
+        .AddOtlpExporter())
+    // OpenTelemetry metrics (S-16c, ADR-0023): the golden signals for the request path —
+    // http.server.request.duration (traffic/errors/latency) + http.client.* for the downstream hops,
+    // plus the built-in System.Runtime meter for saturation (GC, CPU, thread pool). Prometheus scrapes
+    // these from /metrics (mapped below); no OTLP push for metrics, so no collector hop (ADR-0023).
+    .WithMetrics(metrics => metrics
+        .AddAspNetCoreInstrumentation()
+        .AddHttpClientInstrumentation()
+        .AddMeter("System.Runtime")
+        .AddPrometheusExporter());
 
 var keycloakAuthority = builder.Configuration["Keycloak:Authority"]
     ?? throw new InvalidOperationException("Missing configuration 'Keycloak:Authority'");
@@ -82,6 +92,9 @@ app.UseAuthentication();
 app.UseAuthorization();
 
 app.MapHealthChecks("/health");
+
+// Prometheus scrape endpoint (S-16c): exposes the OTel metrics above in Prometheus text format.
+app.MapPrometheusScrapingEndpoint();
 app.MapOpenApi();
 
 // Self-service submit: requires a valid digid token; the bsn comes from the token, not the body,