fix(bitcoind): accept lnbits/lnbits rpcauth alongside cookie

The lnbits regtest test harness (tests/regtest/helpers.py) authenticates to
bitcoind with -rpcuser=lnbits -rpcpassword=lnbits, but bitcoind was cookie-only,
so mine_blocks / pay_onchain / create_onchain_address (and any bitcoind-using
regtest test) failed with "Incorrect rpcuser or rpcpassword".

Add -rpcauth for user lnbits (password lnbits). rpcauth does not set
rpcpassword, so the auto-generated .cookie is still produced and the LN nodes'
--bitcoind.rpccookie auth keeps working. Verified: both cookie and
lnbits/lnbits creds authenticate; wrong creds rejected. Surfaced while
validating aiolabs/lnbits#53 (native on-chain receive/send) against real lnd-1.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -107,6 +107,11 @@ services:
     image: boltz/bitcoin-core:25.0
     command:
       - -regtest
+      # creds for the lnbits regtest test harness (tests/regtest/helpers.py uses
+      # -rpcuser=lnbits -rpcpassword=lnbits). rpcauth keeps the auto-generated
+      # cookie working too, so the LN nodes' --bitcoind.rpccookie auth is intact.
+      # $$ escapes compose interpolation. (user=lnbits, password=lnbits)
+      - -rpcauth=lnbits:a1b2c3d4e5f60718293a4b5c6d7e8f90$$64c29b7500c0e20b7917aa7f6dc6ce3eec896dd8f0bd3834f98c5ee489ef233f
       - -fallbackfee=0.00000253
       - -zmqpubrawtx=tcp://0.0.0.0:29000
       - -zmqpubrawblock=tcp://0.0.0.0:29001