[hf] add make command to fix corrupted postgres write-ahead log

context:
* twice in the last year, we have seen signalboost get in a broken
  state after a database backup job in which it cannot restart because
  it fails to connect to postgres, which itself cannot start because
  of a corrupted write-ahead log
* we would like to be able to fix this problem as quickly as possible
  if it arises again without having to think about things or look them
  up while we are stressed out

changes:
* this MR provides a one-line make command to repair the
  write-ahead-log and is a down payment on eventually automating the fix