Allow using Subgraph API and Omnigraph API only after backfill was completed

[tk-o]

Currently, it's possible for users to interact with Subgraph API and Omnigraph API during backfill. It means means that user queries are executed against partially indexed data. We should only support such queries after the backfill has completed. Therefore, we need to update the UI layer and API layer accordingly.

• ENSAdmin
  • During backfill, for ENS Omnigraph view and ENS Subgraph view, show an error message explaining how those views can only be used after indexing has reached the following status.
• ENSApi
  • During backfill, return 503 Service Unavailable HTTP status for queries to Subgraph API and Omnigraph API.

[coderabbitai]

⚠️ Possible Duplicate Issue(s)

• Refine handling of special indexing status or config states #1446

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