feat(obs): distributed traces across the .NET services (S-16b, closes #123) (#126)

## What & why

S-16b, second of the S-16 split, on top of the #125 backplane. The five .NET services now emit OpenTelemetry traces so a request is **one connected trace** across them.

- Each host wires `AddOpenTelemetry().WithTracing(...)` with `AddAspNetCoreInstrumentation` (incoming) + `AddHttpClientInstrumentation` (outgoing) + `AddOtlpExporter` to **Tempo**.
- Because every cross-service call already goes through a typed `HttpClient` (§8 boundaries), the W3C `traceparent` propagates with no manual code — bff → domain → acl → openzaak and bff → projection-api stitch into a single trace.
- Service name + OTLP endpoint come from `OTEL_*` env set per app service in compose. `/health` is filtered out so liveness polls don't flood the traces.

No new ADR — ADR-0023 already records the stack + the two documented gaps (browser-side tracing is out of scope, so the trace begins at the BFF; the async Flowable-poll boundary is a separate trace).

Closes #123

## Definition of Done

- [x] Failing test committed first (`verify-tracing` fails with no instrumentation).
- [x] Implementation makes it pass — **validated locally end to end**: a real connected trace spanning `bff` + `projection-api` was found in Tempo (BFF→projection→db + Tempo subset, no OpenZaak/egress).
- [x] Conventional Commits referencing the issue (`refs #123`).
- [ ] CI green — awaiting Gitea Actions (verify-tracing added to verify-stack after verify-bff).
- [x] `docker compose up` health unaffected — services boot healthy even when Tempo is unreachable (exporter no-ops; verified).
- [x] Docs — demo-script + BACKLOG.
- [x] ADR — none needed (covered by ADR-0023).

## Notes for reviewers

- **Per-service wiring, no shared lib:** the block is duplicated across the five hosts by design — services don't share code across boundaries here (§8), same as the duplicated typed clients.
- **Packages:** OpenTelemetry.Extensions.Hosting / Instrumentation.AspNetCore / Instrumentation.Http / Exporter.OpenTelemetryProtocol, all 1.17.0, pinned per-csproj (no central props file).
- **The check** generates anonymous BFF→projection traffic (no auth, no OpenZaak), then queries Tempo (TraceQL search → fetch trace → assert both service.names present) from a python:3-slim container in-network — same idiom as run-projection-check.sh.
- **Next:** #124 (S-16c) adds `/metrics` + Prometheus scrape targets + golden-signal Grafana dashboards.

Reviewed-on: #126

infra/docker-compose.yml

@@ -296,6 +296,10 @@ services:
       dockerfile: Dockerfile
     image: register-referentie/acl:dev
     environment:
+      # OpenTelemetry traces → Tempo (S-16b, ADR-0023).
+      OTEL_EXPORTER_OTLP_ENDPOINT: http://tempo:4317
+      OTEL_EXPORTER_OTLP_PROTOCOL: grpc
+      OTEL_SERVICE_NAME: acl
       # Overridable so verify-domain can point the ACL at the same OpenZaak host that
       # owns the seeded zaaktype URL (host-consistent zaak creation, ADR-0009).
       Acl__OpenZaak__BaseUrl: ${ACL_OPENZAAK_BASEURL:-http://openzaak:8000/}
@@ -334,6 +338,10 @@ services:
       dockerfile: Dockerfile
     image: register-referentie/domain:dev
     environment:
+      # OpenTelemetry traces → Tempo (S-16b, ADR-0023).
+      OTEL_EXPORTER_OTLP_ENDPOINT: http://tempo:4317
+      OTEL_EXPORTER_OTLP_PROTOCOL: grpc
+      OTEL_SERVICE_NAME: domain
       Flowable__BaseUrl: http://flowable-rest:8080/flowable-rest/
       Flowable__Username: rest-admin
       Flowable__Password: test
@@ -360,6 +368,10 @@ services:
       dockerfile: Dockerfile
     image: register-referentie/bff:dev
     environment:
+      # OpenTelemetry traces → Tempo (S-16b, ADR-0023).
+      OTEL_EXPORTER_OTLP_ENDPOINT: http://tempo:4317
+      OTEL_EXPORTER_OTLP_PROTOCOL: grpc
+      OTEL_SERVICE_NAME: bff
       # The BFF is the portals' only backend; it validates digid tokens and fans out (ADR-0010).
       # Keycloak (start-dev) derives the issuer from the request host, so the BFF authority and the
       # verify token request both use keycloak:8080 to keep the issuer consistent.
@@ -412,6 +424,10 @@ services:
       dockerfile: services/event-subscriber/Dockerfile
     image: register-referentie/event-subscriber:dev
     environment:
+      # OpenTelemetry traces → Tempo (S-16b, ADR-0023).
+      OTEL_EXPORTER_OTLP_ENDPOINT: http://tempo:4317
+      OTEL_EXPORTER_OTLP_PROTOCOL: grpc
+      OTEL_SERVICE_NAME: event-subscriber
       ConnectionStrings__Projection: Host=projection-db;Database=projection;Username=projection;Password=projection
       # The subscriber enriches the projection with each zaak's reference (identificatie) by asking
       # the ACL — the only code allowed to read ZGW (§8.1, #78).
@@ -441,6 +457,10 @@ services:
       dockerfile: services/projection-api/Dockerfile
     image: register-referentie/projection-api:dev
     environment:
+      # OpenTelemetry traces → Tempo (S-16b, ADR-0023).
+      OTEL_EXPORTER_OTLP_ENDPOINT: http://tempo:4317
+      OTEL_EXPORTER_OTLP_PROTOCOL: grpc
+      OTEL_SERVICE_NAME: projection-api
       ConnectionStrings__Projection: Host=projection-db;Database=projection;Username=projection;Password=projection
     ports:
       - "8120:8080"
@@ -538,12 +558,16 @@ services:
       context: ./observability/tempo
     image: register-referentie/tempo:dev
     command: ["-config.file=/etc/tempo.yaml"]
+    # Cap the backplane's footprint so it can't starve the app stack + the Playwright browser on the
+    # memory-tight CI runner (verify-e2e OOM history, commit d5e5fa2). Generous vs idle (~150M).
+    mem_limit: 400m
     networks: [cg]
 
   prometheus:
     build:
       context: ./observability/prometheus
     image: register-referentie/prometheus:dev
+    mem_limit: 400m
     ports:
       - "9090:9090"
     networks: [cg]
@@ -552,6 +576,7 @@ services:
     build:
       context: ./observability/grafana
     image: register-referentie/grafana:dev
+    mem_limit: 512m
     environment:
       GF_SECURITY_ADMIN_USER: admin
       GF_SECURITY_ADMIN_PASSWORD: admin