subscribe_opml, reachable by anyone signed in through POST /api/opml,
held the catalogue lock while, for each outline, it searched every
subscription and the whole catalogue for its address and probed
unique_slug's suffixes in order. 2 MB holds about 40,000 outlines, so
one upload was quadratic work with every other catalogue change
waiting, and its feeds stayed in the catalogue for good.
opml_feeds lists a document's feeds and refuses more than MAX_LISTED,
the limit a subscribed list already has (#337): a 400 from the web, a
message from `ipx opml`. subscribe_opml takes that list and looks
addresses up in a map built once, the first feed with an address
winning as the search in order did.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>