chore(infra): migrate Jaeger to v2 for built-in dark theme (#64)

## Summary

Jaeger v1's web UI has no theme selector — it ships light-only. v2 (the OTel-Collector-based rewrite, image `jaegertracing/jaeger`, distinct from v1's `jaegertracing/all-in-one`) ships a **Light / Dark / Auto** switcher in the UI nav. v2 is also the actively-developed line; v1 is on the way out within ~6-12 months.

Migration is mechanical:

- Image: `jaegertracing/all-in-one:1.76.0` → `jaegertracing/jaeger:2.17.0`.
- Drop `COLLECTOR_OTLP_ENABLED: 'true'` env — v2 enables OTLP receivers by default.
- UI port (`16686`) and OTLP ports (`4317` / `4318`) are unchanged, so the collector → `jaeger:4317` forwarding pipeline keeps working without touching `otel-collector.yaml`.

## Test plan

- [ ] After merge: `docker compose -f infra/local/dev.compose.yml --profile observability up -d --force-recreate jaeger` → container stays healthy.
- [ ] http://localhost:16686 loads the v2 UI; nav shows the Light/Dark/Auto switcher.
- [ ] Send a synthetic OTLP trace to `localhost:4317` (e.g., via grpcurl or once the BFF is wired in B) → it appears in the Jaeger UI.
- [ ] OTel Collector logs no longer warn abou

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Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #64

infra/local/dev.compose.yml

@@ -133,14 +133,19 @@ services:
   # OTLP ports (4317/4318) are NOT published from this service to
   # avoid colliding with the collector — the collector talks to
   # Jaeger on the internal Compose network.
+  #
+  # Image is Jaeger v2 (`jaegertracing/jaeger`, distinct from the v1
+  # `jaegertracing/all-in-one`). v2 is the rewrite built on top of
+  # OpenTelemetry Collector and is the actively-developed line. OTLP
+  # receivers are enabled by default (no COLLECTOR_OTLP_ENABLED env
+  # needed) and the UI ships a built-in Light/Dark/Auto theme
+  # selector — features the v1 image did not have.
   # ------------------------------------------------------------------
   jaeger:
-    image: jaegertracing/all-in-one:1.76.0
+    image: jaegertracing/jaeger:2.17.0
     container_name: apf-portal-jaeger
     restart: unless-stopped
     profiles: [observability]
-    environment:
-      COLLECTOR_OTLP_ENABLED: 'true'
     ports:
       - '${JAEGER_UI_PORT:-16686}:16686'
     networks: