[CHORE] Drop legacy /index/* redirects (post-0.6.1)

[pjmalandrino]

Context

#225 moved Stores routes from `/index/` to `/ingest/` and added redirect routes so existing bookmarks keep working. The sidebar's `matchPrefixes` was extended to `['/ingest', '/index']` so the nav item stays active during a back-compat redirect.

The redirect is transitional — it should not survive forever:

```ts
// frontend/src/app/router/routes.ts
{ path: '/index', redirect: '/ingest' },
{ path: '/index/new', redirect: '/ingest/new' },
{ path: '/index/:store', redirect: (to) => `/ingest/${to.params.store}` },
{ path: '/index/:store/edit', redirect: (to) => `/ingest/${to.params.store}/edit` },
{ path: '/index/:store/query', redirect: (to) => `/ingest/${to.params.store}/query` },
```

The current commit says "until all caches are warm", with no explicit drop date. This issue tracks the cleanup so the redirects don't linger past 0.7.0.

Scope

• Remove the 5 `/index/*` redirect routes from `frontend/src/app/router/routes.ts`.
• Remove `'/index'` from `matchPrefixes` in `frontend/src/shared/ui/AppSidebar.vue`.
• Update the router tests in `frontend/src/app/router/router.test.ts` to assert the legacy paths now resolve to the 404 / Stores-list catch-all (whichever is current behaviour).
• Verify no remaining references to `/index` strings in the codebase (`rg '/index'`).

Out of scope

• Backend changes — the redirects are purely client-side.
• Renaming the underlying `stores` SQLite table (it's already correctly named).

Acceptance criteria

• Visiting `/index` (and the four sub-paths) no longer hits the redirect — either 404 page or the catch-all is fine, but no `/ingest` rewrite happens.
• Sidebar's `matchPrefixes` contains only `['/ingest']`.
• Router test updated (the legacy paths are explicitly NOT expected to map to a stores route).
• Validation pipeline green.

When to ship

Target: 0.7.0 (the redesign milestone). 0.6.1 already shipped the `/ingest` paths, so anyone hitting a stale bookmark in the 0.7.0 window has had a full minor-version cycle of grace.

References

• [FEAT] Workspace Ingest view + Index→Ingest vocabulary cleanup #225 — the rename that introduced the redirects.
• `frontend/src/app/router/routes.ts:107-130` — the redirect block.
• `frontend/src/shared/ui/AppSidebar.vue:147` — the matchPrefixes list.