[Feature] Phase 0.5: OpenTelemetry logging + metrics + traces (Free, SigNoz)

[PenguinzTech]

All logging AND metrics (and traces) emitted via OpenTelemetry — single OTLP pipeline, ungated/Free tier.

• Backend: OTel SDK + auto-instrumentation (Quart, redis, psycopg, httpx); structlog->OTel logs bridge (keep structlog as dev API); replace Prometheus /metrics with OTel metrics (OTLP push; keep a Prom scrape endpoint only if needed via OTel Prometheus exporter).
• All services (api, worker, scanner, runs-invoker) emit via OTEL_EXPORTER_OTLP_ENDPOINT + per-service OTEL_SERVICE_NAME.
• Export to self-hosted SigNoz (penguin-signoz repo; OTel-native).
• Helm: otel: values block; frontend OTel browser SDK optional.
• Observability is never license-gated.

Spec: docs/design/2026-07-07-elder-platform-merge-design.md (release/v4.0.X)

[PenguinzTech]

Phase 0.5 landed on release/v4.0.X (commit 3c85713). OTel 1.43.0 logs+metrics+traces via OTLP/HTTP to SigNoz, ungated/Free, HTTP-only exporter (no gRPC), protobuf 5.29.6 held, lockfiles hash-pinned. Verified in py3.13 Docker (tests + fail-soft imports + mypy --strict). Auto-closes when release/v4.0.X merges to main.