webhook: skip npm ci when dependency files are unchanged

Deploys ran `npm ci` unconditionally, which wipes node_modules and
rebuilds native modules (better-sqlite3) on every push — ~20 min even
for a frontend-only change with no dependency changes.

Gate the clean install on package.json / package-lock.json actually
changing between the pre-pull HEAD and the new HEAD. As a safety net,
still install whenever the build's required modules (@vitejs/plugin-react,
@rollup/rollup-linux-x64-*) are missing from node_modules, so a skipped
install can never leave an incomplete tree. Frontend-only deploys now
skip straight to the build.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

webhook.cjs

@@ -323,7 +323,63 @@ function runCapture(command, args, options = {}) {
   })
 }
 
-async function ensureBuildDependencies() {
+// Files that, when changed, require a fresh `npm ci`. package.json is included
+// because npm ci refuses to run against a package.json/lockfile that are out of
+// sync, so a change to either should trigger a reinstall.
+const DEPENDENCY_FILES = ['package.json', 'package-lock.json']
+
+// Resolve the modules the build relies on. Returns the first missing module's
+// name, or null if all are present. Used both to validate after an install and
+// to decide whether a skipped install left an incomplete node_modules.
+function missingBuildDependency() {
+  try {
+    require.resolve('@vitejs/plugin-react')
+  } catch {
+    return '@vitejs/plugin-react'
+  }
+
+  if (process.platform === 'linux' && process.arch === 'x64') {
+    const libc = process.report?.getReport?.().header?.glibcVersionRuntime ? 'gnu' : 'musl'
+    const rollup = `@rollup/rollup-linux-x64-${libc}`
+    try {
+      require.resolve(rollup)
+    } catch {
+      return rollup
+    }
+  }
+
+  return null
+}
+
+// True when any dependency file changed between previousHead and the current
+// HEAD. Defaults to true (reinstall) whenever we can't determine the range, so
+// an uncertain state never skips a needed install.
+async function dependencyFilesChanged(previousHead) {
+  if (!validGitSha(previousHead)) return true
+  try {
+    const changed = await runCapture('git', [
+      'diff',
+      '--name-only',
+      `${previousHead}..HEAD`,
+      '--',
+      ...DEPENDENCY_FILES,
+    ])
+    return changed.trim().length > 0
+  } catch {
+    return true
+  }
+}
+
+async function ensureBuildDependencies(previousHead) {
+  const depsChanged = await dependencyFilesChanged(previousHead)
+  // Skip the clean reinstall (which wipes node_modules and rebuilds native
+  // modules like better-sqlite3) only when no dependency files changed AND the
+  // existing node_modules already has everything the build needs.
+  const canSkip = !depsChanged && missingBuildDependency() === null
+
+  if (canSkip) {
+    console.log('deploy step skipped: npm ci (no package.json/package-lock.json changes)')
+  } else {
     await run('npm', ['ci'], {
       env: {
         NODE_ENV: 'development',
@@ -334,18 +390,9 @@ async function ensureBuildDependencies() {
       },
     })
 
-  try {
-    require.resolve('@vitejs/plugin-react')
-  } catch {
-    throw new Error('@vitejs/plugin-react is missing after npm install; dev dependencies were not installed')
-  }
-
-  if (process.platform === 'linux' && process.arch === 'x64') {
-    const libc = process.report?.getReport?.().header?.glibcVersionRuntime ? 'gnu' : 'musl'
-    try {
-      require.resolve(`@rollup/rollup-linux-x64-${libc}`)
-    } catch {
-      throw new Error(`@rollup/rollup-linux-x64-${libc} is missing after npm install; optional dependencies were not installed`)
+    const missing = missingBuildDependency()
+    if (missing) {
+      throw new Error(`${missing} is missing after npm install; dependencies were not installed`)
     }
   }
 }
@@ -596,9 +643,10 @@ async function deploy(push) {
 
   try {
     await notifyDeployStarted(push)
+    const previousHead = await runCapture('git', ['rev-parse', 'HEAD']).catch(() => null)
     await run('git', ['pull', '--ff-only'])
     diff = await deployDiff(push)
-    await ensureBuildDependencies()
+    await ensureBuildDependencies(previousHead)
     await run('npm', ['run', 'build', '--', '--outDir', '../dist-next'])
     validateBuiltDist()
     await run('cargo', ['build', '--manifest-path', 'backend/Cargo.toml', '--release'])