Add system diagnostics health endpoint and UI

[RickyVishwakarma]

What does this PR do?

Adds a system diagnostics feature to make it easier to check if the backend
is ready before running workflows.

Backend

• Extends the existing /health endpoint to report:
  • Database connectivity
  • Model configuration
  • Tool registry availability

Frontend

• Adds a /diagnostics page to show system status
• Protects the page behind authentication
• Uses existing routing and layout

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Why is this needed?

Right now there is no easy way to verify whether the system
is fully set up. This helps with:

• Debugging setup issues
• Onboarding new users
• Understanding runtime state

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How was this tested?

• Checked /health endpoint response
• Loaded diagnostics page when logged in
• Verified redirect to login when logged out