Prevents slow chat auto-archive runs from causing a constant archival
loop by resetting the ticker only after each run completes.
Also documents the UTC midnight cutoff used for archive eligibility so
chats with activity on the same UTC calendar date stay eligible or
ineligible for the full day.
Addresses deferred review comments:
- https://github.com/coder/coder/pull/26109#discussion_r3380197310
- https://github.com/coder/coder/pull/26109#discussion_r3380219922
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