Add whoami-based sync worker routing for user-level sticky sessions

This adds a new routing mechanism for sync workers that resolves access tokens
to usernames via Synapse's whoami endpoint, enabling true user-level sticky
routing regardless of which device or token is used.

Previously, sticky routing relied on parsing the username from native Synapse
tokens (`syt_<base64 username>_...`), which only works with native Synapse auth
and provides device-level stickiness at best. This new approach works with any
auth system (native Synapse, MAS, etc.) because Synapse handles token validation
internally.

Implementation uses nginx's auth_request module with an njs script because:
- The whoami lookup requires an async HTTP subrequest (ngx.fetch)
- js_set handlers must return synchronously and don't support async operations
- auth_request allows the async lookup to complete, then captures the result
  via response headers into nginx variables

The njs script:
- Extracts access tokens from Authorization header or query parameter
- Calls Synapse's whoami endpoint to resolve token -> username
- Caches results in a shared memory zone to minimize latency
- Returns the username via a `X-User-Identifier` header

The username is then used by nginx's upstream hash directive for consistent
worker selection. This leverages nginx's built-in health checking and failover.

roles/custom/matrix-synapse-reverse-proxy-companion/templates/systemd/matrix-synapse-reverse-proxy-companion.service.j2

@@ -36,6 +36,9 @@ ExecStartPre={{ devture_systemd_docker_base_host_command_docker }} create \
 			{% endif %}
 			--mount type=bind,src={{ matrix_synapse_reverse_proxy_companion_base_path }}/nginx.conf,dst=/etc/nginx/nginx.conf,ro \
 			--mount type=bind,src={{ matrix_synapse_reverse_proxy_companion_confd_path }},dst=/etc/nginx/conf.d,ro \
+			{% if matrix_synapse_reverse_proxy_companion_njs_enabled %}
+			--mount type=bind,src={{ matrix_synapse_reverse_proxy_companion_njs_path }},dst=/njs,ro \
+			{% endif %}
 			--label-file={{ matrix_synapse_reverse_proxy_companion_base_path }}/labels \
 			{% for arg in matrix_synapse_reverse_proxy_companion_container_arguments %}
 			{{ arg }} \