feat: add GitHub repository dashboard with Prometheus metrics

ghtracker authenticates as a GitHub App and scans a configured list of repositories on a schedule. It serves a dashboard, a JSON state endpoint, a health probe and Prometheus metrics.

The dashboard lists only items that need attention: failing workflows, pull requests that wait too long or fail their checks, stale issues and projects that failed to scan. It has search, dynamic filters and a button that rescans on demand. The Vue app is embedded in the Go binary.

The client protects the GitHub rate limit with conditional requests, a request reserve and Retry-After handling.

src/lib/api.ts

@@ -0,0 +1,16 @@
+import type { TrackerState } from './types'
+
+export async function fetchState(signal?: AbortSignal): Promise<TrackerState> {
+  const res = await fetch('/api/state', { signal, headers: { Accept: 'application/json' } })
+  if (!res.ok) throw new Error(`state request failed: HTTP ${res.status}`)
+  return (await res.json()) as TrackerState
+}
+
+/** Thrown when the server refuses a rescan because one ran recently. */
+export class RescanLimitedError extends Error {}
+
+export async function requestRefresh(): Promise<void> {
+  const res = await fetch('/refresh', { method: 'POST' })
+  if (res.status === 429) throw new RescanLimitedError('Rescan is limited to once every 5 minutes')
+  if (!res.ok) throw new Error(`rescan request failed: HTTP ${res.status}`)
+}