The client_reader route bucket had collapsed into one long alternation,
which made small worker-audit edits hard to review. Any endpoint change
rewrote the whole regex and obscured whether we were changing routing
policy or just maintaining the route list.

Refactor the variable into grouped regex entries with comments instead.
This keeps the current specialized-worker policy intact: nginx still
renders the client_reader locations in the same block, and the routes
still target the same upstream bucket. The goal here is to make future
doc/code audits, additions, and removals mechanical and reviewable.

This also matches MDAD's current worker model, where generic workers are
not mixed with the specialized room/sync/client/federation reader
routing buckets, so there is no need to derive this from the generic
worker map.

Refs:
- b99a58719b/docs/workers.md (historical-apps)
- b99a58719b/docs/workers.md (synapseappgeneric_worker)