docs(development): add observability dev-loop section (#73)

## Summary

ADR-0012 phase 1 + phase 2 are wired (BFF + SPA), so the "Observability dev-loop" placeholder in the roadmap table now has real content to point at. Promote it from §9 future-work list to a full §5 walkthrough sitting between "Daily commands" and "Dependency updates".

## What §5 covers

- **Bringing up the observability stack** — `./infra/local/dev.sh up observability`, with the endpoint table (Jaeger UI, OTLP receiver, BFF stdout, browser DevTools).
- **Reading a trace in Jaeger** — service-dropdown filter, span attribute keys to look at (`http.method`, `db.statement`, `service.name`, `trace_id`), the trace-id-as-pivot pattern.
- **Correlating a trace with the BFF Pino logs** via `trace_id` and `request_id` (the per-request UUID `nestjs-cls` provides). Concrete `grep` example.
- **Reading SPA spans from the browser** — DevTools Network filter on `localhost:4318/v1/traces` + Jaeger UI cross-check.
- **Common gotchas** — observability profile not active, CORS pre-flight on `/v1/traces`, `propagateTraceHeaderCorsUrls` mismatches, NODE_ENV mis-set, EADDRINUSE on serve restart.
- **What's not in v1** — pointer at the "wired as features land" deltas (CLS keys for session/user/audience, redact list, custom domain spans, prod backend) so a contributor knows what's intentionally not yet there.

## Renumbering

The new §5 pushes existing sections down. Final structure: 1. Repo layout / 2. Prerequisites / 3. Initial setup / 4. Daily commands / 5. **Observability dev-loop** / 6. Dependency updates (Renovate) / 7. Conventional commit cycle / 8. Where to look / 9. Roadmap.

The "Observability dev-loop" line in §9's roadmap table is removed (now implemented).

`docs/README.md` cross-link updated to mention the new section.

## Test plan

- [ ] CI green on this PR (`format:check`).
- [ ] In a fresh checkout, follow §5's "Bring up the observability stack" verbatim, reach the Jaeger UI, then walk a trace through the `grep` correlation example with a synthetic request — flag any step that's wrong / missing on this real-world rehearsal.

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Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #73

docs/README.md

@@ -13,7 +13,7 @@ This is the entry point to all project documentation. It is maintained automatic
 
 ### Daily development
 
-- [development.md](development.md) — repo layout, prerequisites, initial setup, daily commands, dependency updates (Renovate), conventional commit cycle. Day-to-day reference for working on the project.
+- [development.md](development.md) — repo layout, prerequisites, initial setup, daily commands, observability dev-loop (Jaeger UI, log ↔ trace correlation), dependency updates (Renovate), conventional commit cycle. Day-to-day reference for working on the project.
 
 ### Architecture