feat(reconciler): auto-deregister repos delisted from gittensor master list

[plind-junior]

Summary

Add a background reconciler that periodically fetches the canonical gittensor
repo list and automatically sets registered = false for any repo that has
been removed from it.

Motivation

Currently, registered is only flipped to false when the GitHub App is
uninstalled via the installation webhook. If a repo is delisted from
gittensor but the bot stays installed, registered remains true and the
mirror continues persisting webhook events indefinitely — accumulating scoring
data for a repo that is no longer part of the subnet.

The repos table schema already calls this out — registered defaults to
false with a comment: "Manually flipped today; a future reconciler will sync
from on-chain registration."

Proposed Solution

Fetch the canonical repo list from
master_repositories.json
every 2 hours. Any repo in the DB with registered = true whose key is absent
from that file gets flipped to registered = false.

Key design decisions:

• Fail-safe on fetch error — if the HTTP request fails or returns non-200,
  the reconciler logs and exits without touching the DB. A network hiccup will
  never mass-deregister repos.
• installationId is preserved — delisting ≠ uninstalling the bot. If a
  repo is re-listed, the admin can re-register via the existing
  POST /api/v1/admin/repos/register endpoint without a GitHub App reinstall.
• BullMQ repeatable job — runs on the existing github-fetch queue with
  cron 0 */2 * * *. BullMQ deduplicates by repeat key so app restarts don't
  pile up duplicate schedules.
• No new dependencies — uses the existing BullMQ queue, TypeORM, and plain
  fetch.