[rmn-boiko]

Add BTC send and witness receive endpoints

Changes

• POST /wallet/sendbtcbegin with {address, amount, fee_rate} returns {psbt}.
• POST /wallet/sendbtcend with {signed_psbt} broadcasts and returns {txid}.
• POST /wallet/witnessreceive takes the same body as /wallet/blindreceive and returns
  the same ReceiveData. Recipient ids look like bcrt:wvout:…, and the sender funds the
  output via witness_data.amount_sat.
• The receive path now takes a ReceiveKind (Blind / Witness) and calls rgb-lib's
  blind_receive or witness_receive. RgbWalletState::blind_receive_token is renamed
  to receive_token.
• Docs: README API table, CHANGELOG (Unreleased → Added), and the swagger spec, which gains
  the new paths plus SendBtcBeginRequestModel and SendBtcResponse.

Existing routes and response shapes are unchanged.

Notes for review

• Unlike the other begin endpoints, fee_rate on sendbtcbegin has no default. All three
  fields are required.
• sendbtcend uses the shared Psbt request struct, so it also accepts the legacy
  signedPsbt spelling. The spec documents only signed_psbt.