tag v3.6.0: close the write gaps

Mutations that existed in sr.ht's API but that Hutch never called.

Tickets can now be edited and deleted. Tickets and trackers can be subscribed
to and unsubscribed from, reading real state from Ticket.subscription and
Tracker.subscription rather than guessing. Mailing lists can be unsubscribed
from. todo.sr.ht's notifySelf and lists.sr.ht's copySelf are now editable from
Settings instead of only on the web.

mailingListSubscribe is deliberately absent: MailingList exposes no
subscription field, and sr.ht has no discovery API for finding a list you are
not already subscribed to.

SRHTClient dropped from 938 lines to 569. Its five duplicated request paths
collapsed onto shared primitives, and the memory-only cache overload turned out
to be dead alongside executeAndCache.