feat(repos): add install-status endpoint to unblock repo registration

[anderdc]

Problem

The repo registration UI (Step 2 — "Tell us about the repo") verifies that the Gittensor Mirror App is installed on a repo by looking the repo up in GET /api/v1/health's repos[] list.

Since #180 narrowed /health to repos that are installed AND registered = true, a freshly-installed-but-unregistered repo — which is exactly the state the registration page is meant to handle — no longer appears in /health. So the UI wrongly shows "We don't see the Gittensor Mirror App installed on this repository."

This is a chicken-and-egg deadlock: registered only flips to true via the admin register step, but the UI's install gate (which precedes registration) now requires the repo to already be registered. New repos can't get past Step 2.

Fix

Add a small read-only endpoint that answers the question the registration gate actually asks — is the app installed? — independent of registered:

GET /api/v1/repos/:owner/:repo/installation  →  { repo_full_name, installed }

installed is true when an installation row exists with a non-null installation_id. This leaves #180's health-monitoring behavior untouched (health stays "actively tracked repos"); it just restores install-only semantics for the registration gate.

Notes:

• Owner/repo matched case-insensitively, consistent with the installation webhook which stores GitHub's canonical casing (Critical] Silent uninstall failure — data ingestion continues after app removal due to case-sensitive PK match #120).
• Unknown repo returns installed=false, not a 404 (mirrors the maintainers endpoint's convention).

Follow-up

The UI change to call this endpoint instead of scanning /health is in a companion gittensor-ui PR.

Testing

• npm run build, npm run lint, and prettier --check all pass.