fixes: timeAgo utc parse, reload clears unread, sync-event debug

- detail banner timeAgo: SQLite timestamps are UTC without tz marker,
  JS was parsing them as local. Fixed by appending Z when absent —
  same pattern chat uses. "2h ago" for a 30s-old update is gone.
- banner Reload button now re-fires detail.opened so the collab-buckets
  plugin clears the red dot on the board card and calls mark-card-seen.
  Dismiss (×) still leaves the dot — that's the acknowledgement that
  nothing was actually read.
- instrument /api/relay/poll with a sync_debug field so the JS can
  console.debug what happened on the sync-events sub-request (status,
  count, error body). Helps diagnose "no toast" reports.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

public/js/modules/caldav.js

@@ -340,6 +340,9 @@ function showSyncEventToast(ev) {
 }
 
 function drainSyncEvents(relayRes) {
+    if (relayRes?.sync_debug) {
+        console.debug('[sync-events]', relayRes.sync_debug);
+    }
     const events = Array.isArray(relayRes?.sync_events) ? relayRes.sync_events : [];
     for (const ev of events) showSyncEventToast(ev);
 }