docs(obs): add OBSERVABILITY.md developer guide and fix stale env var docs

- New docs/OBSERVABILITY.md: developer-facing guide with a "where to look
  for what" table, common LogQL queries, trace exploration workflow,
  log→trace correlation via traceId links, and a signal summary table
- Link from DEPLOYMENT.md §4 (ops section now points to dev guide) and
  from CLAUDE.md Infrastructure section
- Fix stale DEPLOYMENT.md env var table: OTEL_EXPORTER_OTLP_ENDPOINT
  now documents port 4318 (HTTP) not 4317 (gRPC); add the three new
  env vars wired in this PR (OTEL_LOGS_EXPORTER, OTEL_METRICS_EXPORTER,
  MANAGEMENT_METRICS_TAGS_APPLICATION) with their rationale
- Fix stale obs-tempo service description (port 4318, not 4317)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -274,6 +274,10 @@ Back button pattern — use the shared `<BackButton>` component from `$lib/share
 
 → See [docs/DEPLOYMENT.md](./docs/DEPLOYMENT.md)
 
+## Observability
+
+→ See [docs/OBSERVABILITY.md](./docs/OBSERVABILITY.md) — where to look for logs, traces, metrics, and errors.
+
 ## API Testing
 
 HTTP test files are in `backend/api_tests/` for use with the VS Code REST Client extension.