fix: move extra trailers processing before get_series() for deduplication

Move the extra trailers processing to occur before get_series() so that
extra trailers go through normal deduplication logic, matching the
behavior of other trailers.

Also refactor to use b4.mbox.apply_extra_trailers() helper function
instead of duplicating the logic inline.

Fixes: 96fd3f98 (trailers: add -e flag to apply extra trailers from file)