Add knowledge stats, discourse sync, and tools to dashboard

[neuromechanist]

The dashboard at status.osc.earth/osa shows sync timestamps and usage metrics per community, but is missing:

1. Discourse sync status not shown - all_sync_types omits discourse, so the dashboard never displays Discourse sync info
2. No knowledge base counts - The dashboard shows when things synced but not how many items exist (e.g., GitHub items, docstrings)
3. No available tools listing - Maintainers can't see at a glance what tools their assistant has

Changes

• Add discourse to sync types in backend
• Add KnowledgeStats model to sync status response
• Add available_tools_list to public metrics
• Update dashboard frontend with knowledge stats grid and tools badges