Negative search does not work for headers

A search for `Subject: -keyword` returns the same results as a search for `Subject: keyword`, i.e. the negation indicator is ignored.

The same applies to search for other headers: `Author`, `To`, `Body`

A search for `Message-id` does an exact match (including any leading `-`) - this is OK, as a positive match will in general only match a few messages; negating such a match is not particularly useful.

No file or test is named. Start by locating the search parser and header-matching logic, then reproduce `Subject: -keyword` alongside the positive query for Subject, Author, To, and Body. Done means negative header searches exclude matching messages while Message-id matching, including leading `-`, remains unchanged, with regression tests for these cases.

Written by the indexing model from the issue text.