feat(server): default sync interval to 5m and document evented refresh

[jonnii]

The sync loop was opt-in: STACKIT_SYNC_INTERVAL unset meant 0, which disabled it
entirely, so a server with no explicit interval never pulled remote changes. With
webhooks now able to drive immediate refreshes, the interval becomes the
backstop — so it should exist by default.

• envSyncInterval defaults to 5m when STACKIT_SYNC_INTERVAL is unset; "0" still
  explicitly disables, and an unparseable value falls back to the default rather
  than silently disabling the loop.
• docs/deploy.md: explain the three refresh triggers (interval, webhook, manual),
  document webhook setup (payload URL, secret, push event), the manual-sync
  endpoint, and how local servers stay current without webhooks (fsnotify
  watcher + manual sync). Calls out that GitHub sends no push event for
  refs/stackit/metadata/*, so the interval loop must stay on as a backstop.

---

Stack

• PR refactor(web): remove sample alice/bob stacks from repo provider #1313
  • PR feat(server): drive deploy posture from STACKIT_ENV, not $PORT #1314
    • PR refactor(api): extract Syncer.SyncRepo for per-repo sync triggers #1315
      • PR feat(api): add GitHub webhook signature + push parsing helper #1316
        • PR feat(api): add GitHub webhook receiver for evented refresh #1317
          • PR feat(api): coalesce webhook-driven syncs per repo #1318
            • PR feat(api): add manual sync endpoint for on-demand refresh #1319
              • PR feat(server): default sync interval to 5m and document evented refresh #1320 👈
                • PR feat(server): index repos by owner/repo for GitHub-style routes #1322
                  • PR feat(server): serve repos at /repos/{owner}/{repo} path routes #1323
                  • PR feat(web): GitHub-style path URLs for repos, branches, stacks, PRs #1324

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