Expose catchup_period_seconds in /info JSON (Closes drand#1176)

[alienx5499]

Summary

Adds catchup_period_seconds field to the /info JSON endpoint to help clients, relays, dashboards, and SREs reason about expected pacing during catchup without guessing from network behavior.

Changes

• Added CatchupPeriodSeconds uint32 field to JsonInfoV2 struct in grpc/grpc.go
• Uses protoreflect to safely extract the field from ChainInfoPacket protobuf (backward compatible)
• Field will automatically populate once drand protobuf includes catchup_period_seconds

API Example

{
  "public_key": "<hex>",
  "period": 3,
  "genesis_time": 1692803367,
  "chain_hash": "<hex>",
  "scheme": "bls-unchained-g1-rfc9380",
  "beacon_id": "default",
  "catchup_period_seconds": 1
}

Backward Compatibility

Additive JSON field only. No runtime behavior changes. Works with current protobuf versions (returns 0 if field not present) and will automatically populate once drand protobuf is updated.

Closes drand/drand#1176