btcwallet: add ImportAccountRescan

[losh11]

Change Description

Adds account-aware recovery code to support rescanning historical transactions for imported watch-only accounts. Used by LND WalletKit's ImportAccount.

Currently, ImportAccount creates a watch-only account from an extended public key but cannot detect transactions that occurred before the import. The existing recovery system (used for seed restore) performs gap-limit-based address discovery via FilterBlocks, but is hardcoded to DefaultAccountNum. This PR makes the recovery system account-parameterized so it can be used for any account, and adds ImportAccountWithRescan which imports an account and launches a background recovery scan from a caller-specified birthday height.

This is backend-agnostic — it uses chain.Interface.FilterBlocks() which is implemented by all backends (neutrino, btcd, bitcoind).

A companion PR to lnd will wire these new methods into the ImportAccount RPC by adding birthday_height and recovery_window request fields, allowing users to trigger a historical rescan when importing an account.

Example usage:

$ lncli wallet accounts import --address_type=p2wkh --recovery_window=250 --birthday_height=500000 zpub...

Steps to Test

1. Set up a regtest environment with btcd or bitcoind.
2. Create wallet A, derive addresses from a BIP-84 account, fund them with transactions at various block heights.
3. Export the account's extended public key (zpub).
4. Create wallet B with a separate seed.
5. Call ImportAccountWithRescan on wallet B with the zpub and a birthdayHeight at or before the first funded block.
6. Verify that wallet B discovers the historical transactions and the correct balance is reflected.
7. Verify that spending one of the discovered UTXOs from wallet A is detected by wallet B (outpoint spend monitoring).
8. Test with birthdayHeight set after the funded blocks — verify those transactions are not discovered (as expected).
9. Test gap limit behavior: fund address at index 0 and index 100, import with recoveryWindow=50 (should miss index 100) vs recoveryWindow=150 (should find both).

Pull Request Checklist

Testing

• Your PR passes all CI checks.
• Tests covering the positive and negative (error paths) are included.
• Bug fixes contain tests triggering the bug to prevent regressions.

Code Style and Documentation

• The change is not insubstantial. Typo fixes are not accepted to fight bot spam.
• The change obeys the Code Documentation and Commenting guidelines, and lines wrap at 80.
• Commits follow the Ideal Git Commit Structure.
• Any new logging statements use an appropriate subsystem and logging level.

[yyforyongyu]

Thanks for the PR - just FYI the repo is undergoing a major change, including the rescanning and all the public APIs due to sqlization of the db, for more info check the milestone.