Remove legacy maintainer refs from the UI

## Classification
Task

## Problem

The UI still renders each project's legacy maintainer-reference file as if it
were current linkage, even as projects migrate to
`.project/maintainers.yaml`. Legacy-ref observations remain valuable as
historical evidence in the database, but presenting them as current status is
misleading once a project has adopted the new source.

## Scope

- Audit `web/src/` for surfaces rendering legacy-ref-sourced data: project
page maintainer-file references, observation source labels, any
"sanitize"-era freshness copy.
- Legacy-ref observations should remain in the DB as evidence until the
backend retirement (see relationship below) lands; this issue is only about
the UI no longer presenting them as current.
- Can proceed ahead of the backend retirement — hiding stale UI is
independent of retiring the writer that produces these rows.

## Relationships

- UI-side sibling of a not-yet-filed "retire the legacy maintainer ref
files" backend design draft, which owns the fate of the
`maintainer-sanitize" job and the archiving pipeline more broadly.
- Depends loosely on the sync CronJob rationalisation issue (job topology
determines what "legacy-ref-sourced" means going forward).

Start by auditing web/src/ for project-page maintainer-file references, observation source labels, and any “sanitize”-era freshness copy. Compare the displayed status with the newer .project/maintainers.yaml source. Done means legacy-ref observations remain available as database evidence but are no longer presented in the UI as current linkage or freshness.

Written by the indexing model from the issue text.