Correct OTLP metrics/logs behaviour to 404, and start Phase 2

Alloy only registers an OTLP/HTTP route for a signal with an output, so
POST /v1/metrics and /v1/logs return 404 rather than being accepted and
silently dropped (verified live against otlp.vione.cloud). Fix the claim
everywhere it appeared. Also mark Phase 2 in progress.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/roadmap/phase-5.md

@@ -94,8 +94,8 @@ first), but sequenced after Phase 4 per the roadmap's ordering.
    - `ledgerline.fraud.score` (Histogram; no labels)
    - `ledgerline.stream.lag` (Observable Gauge; label: `consumer_group`)
    - `ledgerline.db.pool.in_use` (Observable UpDownCounter; label: `service`)
-3. **Get your custom metrics somewhere they'll actually land.** Alloy drops OTLP
-   metrics, so pick a path and implement it:
+3. **Get your custom metrics somewhere they'll actually land.** Alloy rejects OTLP
+   metrics (404 on `/v1/metrics`), so pick a path and implement it:
    - *Platform grain:* expose `/metrics` on each service and add scrape jobs to
      `k8s/prometheus/prometheus.yml` (the role's `prometheus_extra_scrape_jobs`),
      then `kubectl apply -k k8s/`. Prometheus scrapes your services directly.