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

Makefile

@@ -43,7 +43,7 @@ export DOCKER_HOST := unix://$(PODMAN_SOCK)
 endif
 endif
 
-.PHONY: ci lint build unit mutation frontend integration verify verify-up verify-acl verify-nrc verify-projection verify-bff verify-domain verify-observability verify-notifications smoke up down local verify-local local-down changelog openzaak-up openzaak-smoke openzaak-seed openzaak-down stack-up stack-smoke stack-down keycloak-up keycloak-smoke keycloak-down flowable-up flowable-smoke flowable-down help
+.PHONY: ci lint build unit mutation frontend integration verify verify-up verify-acl verify-nrc verify-projection verify-bff verify-domain verify-observability verify-tracing verify-notifications smoke up down local verify-local local-down changelog openzaak-up openzaak-smoke openzaak-seed openzaak-down stack-up stack-smoke stack-down keycloak-up keycloak-smoke keycloak-down flowable-up flowable-smoke flowable-down help
 
 ## ci: run the full pipeline — lint, build, unit, mutation, frontend, verify (mirrors Gitea Actions)
 ## `verify` is the live-stack stage (full stack up once → ACL + notification checks).
@@ -175,6 +175,11 @@ verify-e2e:
 verify-observability:
 	bash infra/run-observability-check.sh
 
+## verify-tracing: assert one connected distributed trace spans the .NET services in Tempo
+## (S-16b), against the already-running stack.
+verify-tracing:
+	bash infra/run-tracing-check.sh
+
 ## verify: local mirror of the CI verify-stack job — full stack up once, all checks,
 ## tear down (always). For fast single-concern local iteration use `integration`
 ## (oz-only) or `verify-notifications` (oz+nrc) instead.