fix(health): exclude uninstalled repos from /api/v1/health

[leonaIee]

Summary

GET /api/v1/health returned every row in the repos table, including repos whose GitHub App installation has been removed or uninstalled. Those rows are intentionally kept (soft-cleared to installation_id = NULL, registered = false in installation.handler.ts) so historical scoring evidence isn't lost, but they are no longer tracked. The endpoint's own description advertises "the list of tracked repos", so the inclusion of soft-cleared rows is a contract bug; it also misleads downstream consumers (e.g. gittensor-ui) that read /health as an active-installation signal before allowing registration.

This PR filters listRepoHealth() to installation_id IS NOT NULL using the same Not(IsNull()) idiom already used in fetch.processor.ts. Top-level status, DB/Redis probes, the 503-on-failure behavior, and the stalest-first sort are unchanged. Newly-added-but-not-yet-registered repos (installed but registered = false) remain visible, which is the semantic the UI's "is the App installed?" probe actually needs.

Related Issues

Fixes #132

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Testing

Reproduction steps from the linked issue, executed against a local mirror with the Gittensor Mirror App installed:

1. Install the App on a test repo → row appears in repos with installation_id set; repo is listed in /api/v1/health.
2. Remove the repo from the installation (or uninstall the App) → row remains in repos with installation_id = NULL and registered = false; the repo is no longer in the /api/v1/health repos array.
3. Confirmed unchanged: top-level status: "ok", db / redis sections, 503 behavior when a dep is down, and the stalest-first / nulls-last ordering for the remaining tracked repos.

Build / lint / format on the changed file: npm run build, eslint, and prettier --check all clean.

Checklist

• I have read the Contributing Guide
• Code builds without errors
• New and existing tests pass (if applicable) — packages/das has no automated test suite; manual verification per the steps above (CONTRIBUTING: "Confirm manual testing performed")
• Documentation updated (if applicable) — none required; the existing endpoint description already reads "the list of tracked repos"
• No unnecessary dependencies added