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)

Some client API endpoints (e.g. keys/upload) are backed by Synapse stream writers and
should not rely on broad worker regexes or route-order fallthrough for correctness.

When explicit per-stream routing is missing, requests may be captured by generic, room, or client_reader workers, instead of:
- going to the configured stream writer
- or to `main` when that stream writer is not enabled

This refactors synapse-reverse-proxy-companion's routing so that web-facing stream-backed endpoint families
are handled explicitly and early, with deterministic writer-or-main fallback.

Add first-class support for the missing `device_lists` stream writer,
generalize the same routing model to `push_rules`,
and remove stale broad-route ownership for device-list-sensitive endpoints.

The companion role was tightly coupled to Synapse through shared tags, worker routing, and lifecycle ordering. Keeping them separate added coordination overhead without practical benefits, especially for parallelized execution.

This merges the role into matrix-synapse while keeping companion logic organized under dedicated reverse_proxy_companion task/template subdirectories.

Compatibility is preserved:
- matrix_synapse_reverse_proxy_companion_* variable names remain unchanged
- install/setup companion-specific tags remain available

Cross-role/global wiring is now in group_vars (matrix-synapse section), while role defaults provide sensible standalone defaults and self-wiring for Synapse-owned values.