feat(infra): observability backplane — Tempo + Prometheus + Grafana (S-16a, closes #122) (#125)

## What & why

S-16a, the first of the **S-16 split** (#17 closed → #122/#123/#124, §13). Stands up a local, CI-friendly observability backplane so traces (S-16b) and metrics (S-16c) have somewhere to land, viewable in one Grafana.

- **Grafana Tempo** — OTLP trace ingest (gRPC 4317 / HTTP 4318), local storage.
- **Prometheus** — scrapes itself for now; service `/metrics` targets arrive in S-16c.
- **Grafana** — Tempo + Prometheus datasources auto-provisioned with fixed uids (`tempo`, `prometheus`), exposed on :3000.

All three are small **built images** with config baked in (`infra/observability/`), on the existing `cg` network. **No OTLP collector** (Tempo ingests OTLP directly; Prometheus scrapes) and **no config-volume seeding** — the tools aren't verbatim CG peer modules, so a 3-line `COPY` Dockerfile is the simpler path that still reaches sibling containers on the CI runner (**ADR-0023**).

### Verified, not assumed

`make verify-observability` (new CI `verify-stack` step, run early) asks Grafana to reach both datasources — Prometheus via its health method, Tempo via the datasource proxy (Tempo's plugin implements no health method) — so it proves the datasources are wired, not merely that containers booted. Validated locally against the three containers (no external egress): Grafana healthy, both datasources reachable.

Closes #122

## Definition of Done

- [x] Failing test committed first (`verify-observability` fails with no backplane).
- [x] Implementation makes it pass; verified locally.
- [x] Conventional Commits referencing the issue (`refs #122`).
- [ ] CI green — awaiting Gitea Actions (verify-stack now includes the observability step; `docker compose config` validates locally).
- [ ] `docker compose up` reaches green health within 3 min — new containers are lightweight and off the health-gate list.
- [x] Docs — ADR-0023, demo-script, BACKLOG sync.
- [x] ADR added — `docs/architecture/adr-0023-observability-stack.md`.
- [x] Demo note in `docs/demo-script.md`.

## Notes for reviewers

- **No app changes** — this is pure infra; the five services are untouched (instrumentation is #123/#124).
- **Ports:** Grafana 3000 (admin/admin, anonymous viewer on), Prometheus 9090; Tempo internal to `cg`.
- **CI:** the three containers are added to the failure log-dump list; deliberately **not** added to `WAIT_SVCS` (the check polls Grafana itself, so no in-image healthcheck tool is needed). Trades ~3 small image builds per run.
- **Next:** #123 wires OTLP export + `AddAspNetCoreInstrumentation`/`AddHttpClientInstrumentation` into the five hosts so a request becomes one connected trace in Tempo.

Reviewed-on: #125

BACKLOG.md

@@ -253,10 +253,16 @@ Split (issue #11 closed) into two independently-demoable slices per §13 — the
 
 **Outcome:** Beheer portal lets an admin view ZTC catalogi (read-only first), and manage the ACL's default-fill configuration via a CRUD UI. MFA on the medewerker realm enforced.
 
-### S-16 · OpenTelemetry traces + Grafana dashboard
+### S-16 · OpenTelemetry traces + Grafana dashboard *(split — #17 closed)*
 
 **Outcome:** Traces span portal → BFF → Domain → ACL → OpenZaak and portal → BFF → Domain → Flowable. Grafana dashboards pre-built for golden signals.
 
+Split into independently deployable sub-slices (CLAUDE.md §13):
+
+- **S-16a** (#122) · Observability backplane — Grafana Tempo + Prometheus + Grafana in compose, datasources auto-provisioned (ADR-0023). No collector; config baked into built images.
+- **S-16b** (#123) · Distributed traces across the five .NET services (OTLP → Tempo; nginx `traceparent` passthrough). Depends on S-16a.
+- **S-16c** (#124) · Prometheus metrics + golden-signal Grafana dashboards. Depends on S-16a.
+
 ### S-17 · Quartz.NET scheduler — herregistratie reminder sweep ✅
 
 **Outcome:** Daily Quartz.NET cron job finds inscriptions within 90 days of their herregistratie deadline and reminds each (flag on the aggregate + log). No outbound notification and no domain event in v1 — the reminder is the persisted flag, surfaced on the read model (ADR-0022, #120). Quartz fires time-triggered sweeps; the existing pumps stay as queue-drainers.