feat: auto-generate OpenAPI spec for miner HTTP endpoints

[Tne-max]

Closes #23

This PR fulfills the requested bounty to add an OpenAPI 3 spec for the miner HTTP endpoints.

Changes:

• Adds scripts/generate_openapi.py to programmatically generate an OpenAPI JSON schema from the existing Pydantic models in engram.protocol without rewriting all the �iohttp route handlers.
• Documents the sr25519 signed-challenge header auth scheme.
• Adds docs/openapi.json and docs/index.html (Redoc UI).
• Adds a GitHub Actions workflow (openapi_sync.yml) to ensure the openapi.json spec stays fully synchronized with code changes.

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