Add `ingester.api.root_path` so the HTTP control plane (dashboard + API)
can be reverse-proxied behind a sub-path (e.g. /ingester/) on a shared
origin, instead of needing nginx sub_filter URL-rewriting.
- APIConfig.root_path: normalized ('', or single leading slash, no trailing
slash) via a field_validator; validate_assignment so CLI overrides
normalize the same way as config-file values.
- Forwarded to FastAPI(root_path=) and uvicorn.Config(root_path=) so
OpenAPI/docs links are prefix-aware.
- Dashboard route injects a <base href> matching root_path; all dashboard
fetches are now base-relative, so they resolve under the prefix while
staying identical at the root.
- `serve --root-path` CLI flag.
- Docs: "Behind a reverse proxy" section with an nginx example.
Closes#431
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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