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.

backend/routes/threads.js

@@ -18,6 +18,7 @@ const {
   getUserById,
 } = require('../db');
 const { hasCollectionRole, resolveBoard } = require('./board-access');
+const { recordActivity } = require('../activity');
 
 function resolveAuthorName(reqUser) {
   if (reqUser.display_name || reqUser.username) {
@@ -108,6 +109,14 @@ router.post('/:boardId/threads', (req, res) => {
       });
     }
 
+    recordActivity(req, {
+      boardId: req.params.boardId,
+      action: 'thread.created',
+      targetType: 'thread',
+      targetId: threadId,
+      targetLabel: content.trim().slice(0, 80),
+    });
+
     return res.status(201).json({ thread, comment });
   } catch (err) {
     console.error('[threads] create error:', err);
@@ -144,6 +153,15 @@ router.patch('/:boardId/threads/:threadId', (req, res) => {
       });
     }
 
+    if (status === 'resolved' || status === 'open') {
+      recordActivity(req, {
+        boardId: req.params.boardId,
+        action: status === 'resolved' ? 'thread.resolved' : 'thread.reopened',
+        targetType: 'thread',
+        targetId: req.params.threadId,
+      });
+    }
+
     return res.json({ thread: updated });
   } catch (err) {
     console.error('[threads] status error:', err);
@@ -221,6 +239,14 @@ router.post('/:boardId/threads/:threadId/comments', (req, res) => {
       });
     }
 
+    recordActivity(req, {
+      boardId: req.params.boardId,
+      action: 'comment.added',
+      targetType: 'thread',
+      targetId: req.params.threadId,
+      targetLabel: content.trim().slice(0, 80),
+    });
+
     return res.status(201).json({ comment });
   } catch (err) {
     console.error('[threads] add comment error:', err);