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.

deploy/kubernetes/prometheusrule.yaml

@@ -0,0 +1,35 @@
+apiVersion: monitoring.coreos.com/v1
+kind: PrometheusRule
+metadata:
+  name: ghtracker
+spec:
+  groups:
+    - name: ghtracker
+      rules:
+        - alert: GhtrackerWorkflowFailing
+          expr: ghtracker_workflow_failing == 1
+          for: 10m
+          labels: { severity: warning }
+          annotations:
+            summary: 'Workflow {{ $labels.workflow }} failing in {{ $labels.repo }}'
+        - alert: GhtrackerPullRequestWaiting
+          expr: ghtracker_pr_age_seconds > 3 * 86400
+          labels: { severity: warning }
+          annotations:
+            summary: 'PR #{{ $labels.number }} in {{ $labels.repo }} waiting over 3 days'
+        - alert: GhtrackerRepoDown
+          expr: ghtracker_repo_up == 0
+          for: 30m
+          labels: { severity: warning }
+          annotations:
+            summary: 'ghtracker cannot refresh {{ $labels.repo }}'
+        - alert: GhtrackerDataStale
+          expr: time() - ghtracker_last_refresh_timestamp_seconds > 13 * 3600
+          labels: { severity: warning }
+          annotations:
+            summary: 'ghtracker has not refreshed for over 13 hours'
+        - alert: GhtrackerRateLimitLow
+          expr: ghtracker_github_rate_limit_remaining < 500
+          labels: { severity: warning }
+          annotations:
+            summary: 'GitHub rate limit budget below 500'