fix: Handle Unavailable Bittensor API Data

[RenzoMXD]

Summary

This change fixes issue #7 by adding stable backend error responses and frontend UI states for unavailable Bittensor data.
Previously, if Bittensor chain data, adapter calls, or future indexer requests failed, the backend could return a generic error and the frontend could only show a vague backend-unavailable message.
Now the API returns a typed 503 error with BITTENSOR_DATA_UNAVAILABLE, and the frontend displays a specific message: “Bittensor data is temporarily unavailable.”

Related Issue

Closes: #7

Change Type

• Bug fix
• UI/UX improvement
• Backend/API improvement
• Error handling improvement
• Test coverage
• New feature
• Refactor-only change

Real Behavior Proof

Normal API behavior

curl http://localhost:4100/health

Result:

{"status":"ok","service":"tao-passport-api"}

curl http://localhost:4100/api/passport/sample

Result included:

{
  "walletAddress": "5FAbc123TAOPassportDemoWalletAddress999999999999",
  "trustScore": 89,
  "reputationSignals": [...]
}

Forced Bittensor unavailable behavior

Command used:

TAO_PASSPORT_FORCE_BITTENSOR_UNAVAILABLE=true node backend/dist/src/server.js

curl -i http://localhost:4101/api/passport/sample

Result:

HTTP/1.1 503 Service Unavailable

{
  "error": {
    "code": "BITTENSOR_DATA_UNAVAILABLE",
    "message": "Bittensor data is temporarily unavailable. Please retry after the chain adapter reconnects.",
    "retryable": true,
    "source": "bittensor"
  }
}

The same typed error behavior was also verified for:

curl -i http://localhost:4101/api/reputation/leaderboard

Checklist

• Checked issue comments and linked discussion
• Confirmed no issue comments were present
• Added typed backend API error shape
• Added Bittensor unavailable error handling
• Returned stable 503 response for unavailable Bittensor data
• Updated frontend API error parsing
• Added specific unavailable-data UI message
• Preserved existing passport data when refresh fails
• Added backend test coverage
• Ran build
• Manually verified backend normal path
• Manually verified backend unavailable-data path
• Manually verified frontend preview