fix: 15 bug fixes for v3.1.0 dashboard and Track E features

Bug fixes (ordered by priority):
- BUG-14 (P1): Fix ImportError in per-user MCP endpoint
- BUG-8 (P2): Raise clear error when PUBLIC_URL missing for OAuth
- BUG-12 (P3): Validate ENCRYPTION_KEY at startup
- BUG-13 (P4): Handle stale JWT gracefully on site creation
- BUG-11 (P5): Add auth validation to WordPress health checks
- BUG-15 (P6): Fix copy button showing Python repr (dict method shadow)
- BUG-1 (P7): Dynamic version from pyproject.toml instead of hardcoded
- BUG-5 (P8): Fix server mode default (sse → streamable-http)
- BUG-9 (P9): Plugin-specific example tools in endpoint instructions
- BUG-4 (P10): Fix settings page sidebar RBAC (session type mismatch)
- BUG-6 (P11): Show real plugin descriptions instead of 'No description'
- BUG-2 (P12): Remove stale 'phase: X.3' from system info
- BUG-10 (P13): Guard timestamps against None in Recent Activity
- BUG-3 (P14): Guard timestamps against None in audit logs
- BUG-7 (P15): Styled 404 page instead of plain text

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/templates/dashboard/index.html

@@ -140,7 +140,7 @@
                                     <p class="text-xs text-gray-500 dark:text-gray-400">{{ activity.project }}</p>
                                 </div>
                             </div>
-                            <span class="text-xs text-gray-400 dark:text-gray-500">{{ activity.timestamp[:19] }}</span>
+                            <span class="text-xs text-gray-400 dark:text-gray-500">{{ activity.timestamp[:19] if activity.timestamp else '-' }}</span>
                         </div>
                     </div>
                     {% endfor %}