Feat v0.6.15 user display audit

Add batch user resolve endpoint, sw.users SDK module, and migrate all
surfaces to show display_name instead of UUIDs. Canonical fallback chain:
display_name → username → "Unknown".

- GET /api/v1/users/resolve?ids=... (max 100, map response)
- sw.users.resolve(), resolveMany(), displayName() with 60s cache
- Chat participants resolved from users table, not snapshot
- Admin users/teams/groups/team-admin show display_name
- Chat-core participants.display_name column deprecated
- 5 new handler tests, OpenAPI spec updated

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

packages/team-activity-log/js/main.js

@@ -181,7 +181,7 @@
           <div key=${e.id} class="ext-team-activity-log-entry">
             <div class="ext-team-activity-log-entry__header">
               <span class="ext-team-activity-log-entry__category badge">${e.category}</span>
-              <span class="ext-team-activity-log-entry__user">${esc(e.username || 'unknown')}</span>
+              <span class="ext-team-activity-log-entry__user">${esc(e.username || 'Unknown')}</span>
               <span class="ext-team-activity-log-entry__time">${timeAgo(e.created_at)}</span>
               <button class="ext-team-activity-log-entry__delete" onClick=${() => onDelete(e.id)}
                       title="Delete">×</button>