* perf(channels): batch user lookup in model_response_handler thread history
The thread-history builder in model_response_handler called
Users.get_user_by_id once per thread message (deduped via an intra-loop
dict), producing N individual SELECTs for a thread of N unique authors.
Replace with a single Users.get_users_by_user_ids call that returns all
authors in one WHERE id IN (...) query, matching the batch pattern
already used elsewhere in this file (lines 739, 804, 1320).
Behavior is preserved: deleted users still resolve to None and fall
through to the existing 'Unknown' fallback via .get().
* refac(channels): rename loop vars to full words per review
Address reviewer feedback to use descriptive names `message` and `user`
instead of single-letter `m` and `u` in the batch user-lookup
comprehensions.
---------
Co-authored-by: Claude <noreply@anthropic.com>
