Add `thread_subscriptions` as a supported web-facing stream writer in MDAD and
route its unstable client endpoints via the same explicit writer-or-main model
used for the other web-facing stream-backed APIs.

This is not just another generic worker route. Current Synapse gives thread
subscriptions their own `writers.thread_subscriptions` configuration, backs them
with a multi-writer stream, and asserts on store writes that the current
instance is an allowed thread-subscriptions writer.

Explicit early routing is also required here because the subscription endpoint is
room-scoped. In MDAD's specialized-worker model, the existing room-worker regex
would otherwise match `/_matrix/client/unstable/io.element.msc4306/rooms/...`
and steal the request before it reached the correct writer-or-main fallback.

Unlike `device_lists`, support is added without enabling a thread-subscriptions
worker by default in the standard presets. The underlying MSC4306/4308 feature
remains unstable and disabled by default upstream, so the conservative default
is to keep the worker count at `0` and let the new explicit routes fall back to
`main` unless an operator opts in.

Refs:
- b99a58719b/synapse/config/workers.py (L175-L182)
- b99a58719b/synapse/rest/client/thread_subscriptions.py (L38-L247)
- b99a58719b/synapse/storage/databases/main/thread_subscriptions.py (L66-L83)
- b99a58719b/synapse/storage/databases/main/thread_subscriptions.py (L192-L322)