feat: per-board activity log

Adds an audit trail per board, visible from a new clock-icon button on the
toolbar. Useful for review-style work where someone wants to see who
contributed which references and when.

Logged events (high-signal only — canvas-edit noise intentionally skipped):
- image / video / pdf added (whether dropped, pasted, or pulled from a URL)
- board created / renamed / deleted
- thread started, resolved, reopened
- comment posted on a thread

Backend:
- new activity_logs table (id, board_id, user_id, denormalised actor name +
  email, action, target_type/id/label, metadata JSON, created_at) with an
  index on (board_id, created_at DESC).
- logActivity helper resolves the user once at log time and stores their
  display name + email so entries survive deactivation/rename.
- recordActivity wraps logActivity + a Socket.IO emit to the board's room
  so the panel updates live without polling.
- GET /api/boards/:id/activity?limit=&before= for pagination
  (collection-membership gated, viewer+).

Frontend:
- ActivityPanel side-drawer: time-grouped feed (Today / Yesterday / older),
  per-action icons + tone colours (add/remove/edit/comment), pagination
  via "Load older", live append on Socket.IO 'activity:new'.
- Relative timestamps refresh every 30s.
- Wired into Editor + Toolbar.

README updated; roadmap entry checked off.

frontend/src/pages/Editor.tsx

@@ -25,6 +25,7 @@ import { getStickyWidthForSize } from '../canvas/stickyPresets';
 import VideoControls from '../components/VideoControls';
 import ShortcutsHelp from '../components/ShortcutsHelp';
 import Minimap from '../components/Minimap';
+import ActivityPanel from '../components/ActivityPanel';
 import UploadPanel from '../components/UploadPanel';
 import ExportDialog from '../components/ExportDialog';
 import FeedbackPanel from '../components/feedback/FeedbackPanel';
@@ -115,6 +116,7 @@ export default function Editor({ isPublicView }: EditorProps) {
   const [contextMenu, setContextMenu] = useState<{ x: number; y: number } | null>(null);
   const [toasts, setToasts] = useState<{ id: string; text: string }[]>([]);
   const [showLayers, setShowLayers] = useState(false);
+  const [showActivity, setShowActivity] = useState(false);
   const [showGrid, setShowGrid] = useState(true);
   const [showHelp, setShowHelp] = useState(false);
   const [showExport, setShowExport] = useState(false);
@@ -1090,6 +1092,8 @@ export default function Editor({ isPublicView }: EditorProps) {
         followingUserId={followingUserId}
         onToggleLayers={() => setShowLayers((v) => !v)}
         showLayers={showLayers}
+        onToggleActivity={() => setShowActivity((v) => !v)}
+        showActivity={showActivity}
         onToggleHelp={() => setShowHelp((v) => !v)}
         onExport={() => setShowExport(true)}
         onRefreshPreview={readOnly || isPublicView ? undefined : handleRefreshPreview}
@@ -1539,6 +1543,11 @@ export default function Editor({ isPublicView }: EditorProps) {
           />
         )}
 
+        {/* Activity log */}
+        {showActivity && resolvedBoardId && (
+          <ActivityPanel boardId={resolvedBoardId} onClose={() => setShowActivity(false)} />
+        )}
+
         {/* Layer panel */}
         {showLayers && (
           <LayerPanel