Cache-busting version stamps + client-side encounter version tracking

1. Build-ID cache busting (server.js):
   - Compute a BUILD_ID at boot: git HEAD short hash if available,
     else /app/BUILD_ID file, else random-on-boot.
   - On first request for /, rewrite every local /js/*.js and
     /css/*.css reference in index.html to include ?v=BUILD_ID.
     Cached once at startup so subsequent renders are free.
   - X-Build-Id response header + GET /api/build expose it for
     debugging.
   - Eliminates the "works after hard-refresh" class of bugs: every
     deploy gets a new build ID, so browsers fetch fresh JS/CSS on
     the very next page load.

2. Optimistic encounter locking wired into the client
   (public/js/encounters.js):
   - On resumeEncounter(): stash enc.version into
     window._encounterVersions[id]
   - On saveEncounter(): send expected_version in the POST body
     when we have one.
   - Server returns 409 if another tab/device wrote first → user
     sees "Someone else edited this encounter. Reload to see the
     latest version." instead of silently clobbering the prior save.
   - On success, remember the new server-assigned version for the
     next save.

public/js/encounters.js

@@ -117,16 +117,31 @@
     // Prevent duplicate saves from double-click
     if (_savingInProgress[type]) { showToast('Save in progress…', 'info'); return; }
     _savingInProgress[type] = true;
+    // Optimistic locking: send the version we last saw. Server returns 409
+    // if the encounter was updated elsewhere (another tab, another device).
+    var body = Object.assign({}, opts);
+    if (opts.id && window._encounterVersions && window._encounterVersions[opts.id] != null) {
+      body.expected_version = window._encounterVersions[opts.id];
+    }
     fetch('/api/encounters/saved', {
       method: 'POST',
       headers: getAuthHeaders(),
-      body: JSON.stringify(opts)
+      body: JSON.stringify(body)
     })
-    .then(function(r) { return r.json(); })
+    .then(function(r) { return r.json().then(function(d) { d._status = r.status; return d; }); })
     .then(function(data) {
       _savingInProgress[type] = false;
+      if (data._status === 409) {
+        showToast('Someone else edited this encounter. Reload to see the latest version.', 'error');
+        return;
+      }
       if (data.success) {
         showToast('Encounter saved (' + (opts.label || '') + ')', 'success');
+        // Track the new version the server assigned
+        if (data.id != null && data.version != null) {
+          window._encounterVersions = window._encounterVersions || {};
+          window._encounterVersions[data.id] = data.version;
+        }
         // Persist ID so refreshing the page won't create a duplicate
         if (data.id) {
           if (opts.onSaved) opts.onSaved(data.id);
@@ -275,6 +290,11 @@
       .then(function(data) {
         if (!data.success) { showToast(data.error || 'Failed', 'error'); return; }
         var enc = data.encounter;
+        // Remember the loaded version so future saves can detect drift
+        if (enc && enc.id != null && enc.version != null) {
+          window._encounterVersions = window._encounterVersions || {};
+          window._encounterVersions[enc.id] = enc.version;
+        }
         // Close all popovers
         ['enc', 'dict', 'wv', 'sick', 'hosp', 'chart', 'soap'].forEach(function(p) {
           var pop = document.getElementById(p + '-load-popover');

server.js

@@ -130,6 +130,55 @@ app.get('/.well-known/assetlinks.json', (req, res) => {
   res.sendFile(path.join(__dirname, 'public', '.well-known', 'assetlinks.json'));
 });
 
+// ============================================================
+// CACHE-BUSTING VERSION STAMP
+// ============================================================
+// Compute a per-boot BUILD_ID (short hex). Inject it as ?v=BUILD_ID
+// on every local /js/*.js and /css/*.css reference in index.html so
+// browsers always fetch fresh JS/CSS after a deploy instead of
+// serving from the 1-hour cache.
+var fs = require('fs');
+var crypto = require('crypto');
+var BUILD_ID = crypto.randomBytes(4).toString('hex');
+try {
+  var gitHead = fs.readFileSync(path.join(__dirname, '.git/HEAD'), 'utf8').trim();
+  if (gitHead.indexOf('ref:') === 0) {
+    var refPath = gitHead.split(' ')[1];
+    BUILD_ID = fs.readFileSync(path.join(__dirname, '.git', refPath), 'utf8').trim().slice(0, 7);
+  } else {
+    BUILD_ID = gitHead.slice(0, 7);
+  }
+} catch (e) {
+  // Non-git environment (Docker image) — use /app/BUILD_ID file if present,
+  // otherwise stick with the random-on-boot value generated above.
+  try {
+    BUILD_ID = fs.readFileSync(path.join(__dirname, 'BUILD_ID'), 'utf8').trim() || BUILD_ID;
+  } catch (_) {}
+}
+console.log('🔖 Build ID:', BUILD_ID);
+
+// Template index.html once at boot with the version stamp. Serves fast
+// because the rewrite happens one time and the string is reused.
+var INDEX_HTML_TEMPLATED = null;
+try {
+  var raw = fs.readFileSync(path.join(__dirname, 'public', 'index.html'), 'utf8');
+  INDEX_HTML_TEMPLATED = raw.replace(
+    /(<(?:script|link)[^>]+(?:src|href)=["'])(\/(?:js|css)\/[^"'?]+)(["'])/g,
+    '$1$2?v=' + BUILD_ID + '$3'
+  );
+} catch (e) { console.warn('[build-id] index.html template skipped:', e.message); }
+
+app.get(['/', '/index.html'], function(req, res) {
+  if (!INDEX_HTML_TEMPLATED) return res.sendFile(path.join(__dirname, 'public', 'index.html'));
+  res.setHeader('Content-Type', 'text/html; charset=utf-8');
+  res.setHeader('Cache-Control', 'no-cache, no-store, must-revalidate');
+  res.setHeader('X-Build-Id', BUILD_ID);
+  res.send(INDEX_HTML_TEMPLATED);
+});
+
+// Public endpoint for cache-bust debugging + build-info
+app.get('/api/build', function(req, res) { res.json({ buildId: BUILD_ID }); });
+
 app.use(loggingMiddleware);
 app.use(express.static(path.join(__dirname, 'public'), {
   setHeaders: (res, filePath) => {