build-logs: SSE live streaming + raw download (#726 phase 4)

Backend (hive-c0re):
- build_logs.rs: add tokio::sync::broadcast::Sender<i64> to BuildLogs;
  append() and finish() notify subscribers after each sqlite write.
  Add BuildLogProgress struct and get_progress(id, stdout_cursor,
  stderr_cursor) for incremental delta reads.
- dashboard.rs: two new endpoints —
    GET /api/build-logs/id/{id}/stream  SSE; streams BuildLogFrame
      {stdout_append, stderr_append, status?, done} deltas until the
      build finishes or the browser disconnects. Backed by an mpsc
      channel task that watches the per-build broadcast notifications.
    GET /api/build-logs/id/{id}/raw    text/plain download with
      Content-Disposition: attachment; filename build-log-{agent}-{id}.txt

Frontend (dashboard):
- tabs.js: running builds (status === null) connect an EventSource to
  /stream and append lines live; "live" badge pulses amber while active,
  flips to ok/fail on done. Finished builds still use the JSON fetch path.
  Collapsing a running panel closes the EventSource; re-expanding
  reconnects. Adds a "⬇ raw" download link to every expanded row.
- dashboard.css: .build-logs-dl inline download link; .build-logs-live
  live pulse @keyframes animation.

Docs: web-ui.md updated for all three new endpoints + behaviour.

frontend/packages/dashboard/src/dashboard.css

@@ -587,6 +587,28 @@ a:hover {
 .badge-ok      { background: rgba(166,227,161,0.12); color: var(--green);  border-color: var(--green); }
 .badge-fail    { background: rgba(243,139,168,0.12); color: var(--red);    border-color: var(--red);   }
 .badge-running { background: rgba(250,179,135,0.12); color: var(--amber);  border-color: var(--amber); }
+/* Download link sits inline after the row button. Shown only while
+   the detail is expanded (toggled by JS). */
+.build-logs-dl {
+  display: none;
+  font-size: 0.72em;
+  padding: 0.15em 0.45em;
+  margin-left: 0.3em;
+  color: var(--muted);
+  text-decoration: none;
+  border: 1px solid var(--border);
+  border-radius: 3px;
+}
+.build-logs-dl:not([hidden]) { display: inline-block; }
+.build-logs-dl:hover { color: var(--fg); border-color: var(--purple-dim); }
+/* Live-streaming indicator badge inside the detail pane header. */
+.build-logs-live-badge { margin-bottom: 0.4em; }
+/* Pulse animation on the "live" badge text while streaming. */
+.build-logs-live-badge.badge-running { animation: live-pulse 1.4s ease-in-out infinite; }
+@keyframes live-pulse {
+  0%, 100% { opacity: 1; }
+  50%       { opacity: 0.45; }
+}
 
 /* Notification controls — sit between the banner and the
    containers section. Hidden by JS when notifications are

frontend/packages/dashboard/src/tabs.js

@@ -1134,24 +1134,44 @@ window.marked = marked;
             );
             const detail = el('div', { class: 'build-logs-detail' });
             detail.hidden = true;
+            // `loaded` stays false for running builds until SSE
+            // signals done — re-collapsing a running panel stops the
+            // stream and re-expanding reconnects it.
             let loaded = false;
+            // Download link always points at the raw-text endpoint;
+            // hidden until the row is expanded for the first time.
+            const dlLink = el('a', {
+              href: '/api/build-logs/id/' + h.id + '/raw',
+              download: 'build-log-' + h.id + '.txt',
+              class: 'build-logs-dl',
+              hidden: '',
+            }, '⬇ raw');
             rowBtn.addEventListener('click', async () => {
               if (!detail.hidden) {
+                // Collapse: hide panel, close any live SSE stream.
                 detail.hidden = true;
+                dlLink.hidden = true;
                 rowBtn.setAttribute('aria-expanded', 'false');
+                if (detail._es) { detail._es.close(); detail._es = null; }
                 return;
               }
-              if (!loaded) {
+              // Expand
+              detail.hidden = false;
+              dlLink.hidden = false;
+              rowBtn.setAttribute('aria-expanded', 'true');
+              if (loaded) return; // finished build, cached content ready
+
+              if (h.status) {
+                // ── finished build: fetch full JSON once ──────────────
                 detail.textContent = 'fetching…';
-                detail.hidden = false;
-                rowBtn.setAttribute('aria-expanded', 'true');
                 try {
                   const r2 = await fetch('/api/build-logs/id/' + h.id);
                   if (!r2.ok) {
                     detail.textContent = 'error ' + r2.status + ': ' + await r2.text();
                   } else {
                     const full = await r2.json();
-                    const out = (full.stdout || '') + (full.stderr ? '\n--- stderr ---\n' + full.stderr : '');
+                    const out = (full.stdout || '')
+                      + (full.stderr ? '\n--- stderr ---\n' + full.stderr : '');
                     const pre = el('pre', { class: 'build-logs-output' }, out || '(empty)');
                     detail.replaceChildren(pre);
                     loaded = true;
@@ -1160,11 +1180,49 @@ window.marked = marked;
                   detail.textContent = 'fetch failed: ' + err;
                 }
               } else {
-                detail.hidden = false;
-                rowBtn.setAttribute('aria-expanded', 'true');
+                // ── running build: stream via SSE ─────────────────────
+                const pre = el('pre', { class: 'build-logs-output build-logs-live' }, '');
+                detail.replaceChildren(
+                  el('span', { class: 'build-logs-live-badge badge badge-running' }, 'live'),
+                  pre,
+                );
+                let stdoutLen = 0;
+                let stderrLen = 0;
+                const es = new EventSource('/api/build-logs/id/' + h.id + '/stream');
+                detail._es = es;
+                es.onmessage = (ev) => {
+                  let frame;
+                  try { frame = JSON.parse(ev.data); } catch { return; }
+                  if (frame.stdout_append) {
+                    pre.textContent += frame.stdout_append;
+                    stdoutLen += frame.stdout_append.length;
+                  }
+                  if (frame.stderr_append) {
+                    if (stderrLen === 0) pre.textContent += '\n--- stderr ---\n';
+                    pre.textContent += frame.stderr_append;
+                    stderrLen += frame.stderr_append.length;
+                  }
+                  if (frame.done) {
+                    es.close();
+                    detail._es = null;
+                    // Replace live badge with final status
+                    const badge = detail.querySelector('.build-logs-live-badge');
+                    if (badge) {
+                      badge.className = frame.status === 'ok'
+                        ? 'badge badge-ok' : 'badge badge-fail';
+                      badge.textContent = frame.status || 'done';
+                    }
+                    loaded = true;
+                  }
+                };
+                es.onerror = () => {
+                  if (es.readyState === EventSource.CLOSED) {
+                    detail._es = null;
+                  }
+                };
               }
             });
-            li.append(rowBtn, detail);
+            li.append(rowBtn, dlLink, detail);
             list.append(li);
           }
         } catch (err) {