Add a dedicated health endpoint for the Matrix frontend to check backend
capabilities and availability.
Changes:
- Added GET /api/matrix/health endpoint in src/dashboard/routes/world.py
- Returns status (ok/degraded), version, and capabilities dict
- Capabilities: thinking, memory, bark, familiar, lightning
- Each capability checks if the relevant module is available
- Response time <100ms (lightweight checks)
- Returns 200 even if some capabilities are degraded
- Added comprehensive tests in tests/dashboard/test_world_api.py
Fixes#685
