fix(kb): add ChromaDB service monitoring to health checks and API (#3461)

[mrveiss]

Problem: ChromaDB (Knowledge Base) availability was not monitored by health checks or the service status API, making KB outages invisible to the frontend.

Root Cause:

• Ansible health-check playbook did not check autobot-chromadb service or port 8100
• Backend service_monitor.py API listed Redis/Ollama/NPU/Browser but omitted AI Stack entirely

Solution:

1. Ansible health-check.yml: Add autobot-chromadb service check and health endpoint test (port 8100)
2. Backend service_monitor.py: Add ChromaDB health checks to both get_service_statuses() and get_vm_statuses() endpoints

Result:

• Frontend health widget now shows ChromaDB status alongside other services
• Health-check playbooks validate ChromaDB is running before assuming KB is available
• Issue fix(kb): ChromaDB not deployed — knowledge base unavailable on start #3461 resolved

Closes #3461