Add commits-behind count to update status

Calls Forgejo compare API when an update is available to get the exact
number of commits between deployed and latest — exposed as commitsBehind
in the status payload. Degrades gracefully to null if the compare call fails.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

updater/src/index.js

@@ -162,6 +162,19 @@ async function getForgejoCommit(repo) {
   }
 }
 
+async function getCommitsBehind(repo, baseSha, headSha) {
+  try {
+    const headers = { Accept: 'application/json' }
+    if (FORGEJO_TOKEN) headers['Authorization'] = `token ${FORGEJO_TOKEN}`
+    const res = await fetch(`${FORGEJO_URL}/api/v1/repos/${FORGEJO_ORG}/${repo}/compare/${baseSha}...${headSha}`, { headers })
+    if (!res.ok) return null
+    const data = await res.json()
+    return typeof data.total_commits === 'number' ? data.total_commits : null
+  } catch {
+    return null
+  }
+}
+
 async function refreshStatus(repo) {
   const target = getDeployTarget(repo)
   if (!target) return null
@@ -178,6 +191,8 @@ async function refreshStatus(repo) {
     getForgejoCommit(repo),
   ])
   const deployed = !!currentSha
+  const updateAvailable = !!(deployed && latest && currentSha && latest.sha !== currentSha)
+  const commitsBehind = updateAvailable ? await getCommitsBehind(repo, currentSha, latest.sha) : null
   const entry = {
     repo,
     host: target.host,
@@ -185,7 +200,8 @@ async function refreshStatus(repo) {
     currentCommitAt: currentTimestamp || null,
     latestCommit: latest ? latest.sha.slice(0, 12) : 'unknown',
     latestCommitAt: latest?.timestamp || null,
-    updateAvailable: !!(deployed && latest && currentSha && latest.sha !== currentSha),
+    updateAvailable,
+    commitsBehind,
     deployed,
     checkedAt: new Date().toISOString(),
   }