Append monthly files under a declared schema

The rules

These rules decide what a correct result is. Your answer is checked on this dataset and on hidden datasets that follow the same rules.

Inputs (three files, all text)
  jan(id, amount)            feb(id, amount, channel)            bad(id, currency)

Rules
  Schema: required columns id and amount; optional column channel. No other columns are allowed.
  A file missing a required column, or containing a column outside the schema, is rejected as a whole.
  Keep every row of accepted files. A missing optional column gives a missing channel.
  source_file is the name of the file the row came from (jan, feb, …).
  amount is always a whole number in accepted files.

Output (row order does not matter)
  clean(id text, amount int64, channel text, source_file text)