The account condition field lumped in with folder and compared against
email.folder_id, so WHERE account = '...' would false-positive on any
message sharing a folder with the target account. Split the arm so it
reads email.account_id instead.
header[...] conditions silently evaluated to false forever because Email
has no parsed-headers field to match against. Reject header conditions
at validation time with an honest FieldTypeMismatch instead of accepting
a rule that can never fire; the runtime fallback is now documented as an
unreachable defensive branch.
Refs #191
James Maes