Reference: snowbird readiness notes

[tripledoublev]

• This PR is a reference patch based on findings from the DWeb issues document and recent save-dweb-backend and save-rust updates.

• This PR is intentionally a reference implementation for review and adaptation by the Android team.

  • Improves Snowbird startup readiness checks:
    • Requires both liveness and backend readiness before marking connected.
    • Uses /status + /health/ready polling path.
    • Keeps retry behavior for transient startup failures.
  • Adds request guardrails while service is not ready:
    • Group/repo/file repositories now short-circuit with a clear “server not ready” error when service is not connected.
  • Improves refresh correctness and diagnostics:
    • Filters refresh results to repos scoped to the selected group.
    • Classifies repo refresh failures into buckets (DHT_DISCOVERY, PEER_DOWNLOAD, UNKNOWN) for clearer troubleshooting.
  • Aligns upload response handling with latest Rust response shape:
    • Supports optional file_hash in upload result and uses it when present.
  • Adds concise lifecycle reference doc:
    • docs/SnowbirdLifecycle.md summarizing startup flow, readiness checks, and refresh guardrails.