The whoami-based approach is now the only implementation for sync worker routing.
It works with all token types (native Synapse, MAS, etc.) and is automatically
enabled when sync workers exist.

The old map-based approach only worked with native Synapse tokens (syt_<b64>_...)
and would give poor results with MAS or other auth systems.

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.

Synapse Admin v0.11.1-etke53

The /_matrix/client/unstable/org.matrix.simplified_msc3575/sync endpoint
can be handled by generic workers, but Synapse's workers.md documentation
doesn't mention it. The code confirms it's worker-compatible:

- SlidingSyncRestServlet is registered via sync.register_servlets:
  https://github.com/element-hq/synapse/blob/0dfcffab0f/synapse/rest/client/sync.py#L1128-L1131

- sync.register_servlets is NOT in the worker exclusion list:
  https://github.com/element-hq/synapse/blob/0dfcffab0f/synapse/rest/__init__.py#L180-L194

- GenericWorkerStore includes SlidingSyncStore:
  https://github.com/element-hq/synapse/blob/0dfcffab0f/synapse/app/generic_worker.py#L168

This adds the endpoint to both:
- matrix_synapse_workers_sync_worker_client_server_endpoints (for specialized sync workers with sticky routing)
- matrix_synapse_workers_generic_worker_endpoints (documenting generic worker capability)

- Upgrade from 1.0.1 to 4.0.0
- Add ircService.mediaProxy configuration for authenticated Matrix media
- Add Traefik integration for media proxy endpoint
- Generate signing key for authenticated media

Closes #3512

Co-authored-by: Jade Ellis <jade@ellis.link>
Co-authored-by: Slavi Pantaleev <slavi@devture.com>

Ref: https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/4880#issuecomment-3807433691

Ref:
- https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/4882
- https://github.com/element-hq/synapse/pull/19204
- https://element-hq.github.io/synapse/latest/usage/configuration/config_documentation.html#enable_local_media_storage

We default it to `true`, keeping up with upstream and the old behavior.

s3-storage-provider users may set `matrix_synapse_enable_local_media_storage` to `false`
to disable local file caching.
This likely comes at the expense of some performance.

For matrix-media-repo users, it likely doesn't matter what this is set to,
as for a matrix-media-repo setup, all media-related API endpoints are
captured and forwarded to matrix-media-repo (before reaching Synapse).

This adds the matrix_hookshot_connections variable for configuring
static webhook connections via the config file.

See: https://github.com/matrix-org/matrix-hookshot/pull/1102

All these `encodedCharacters_*` settings default to `true` in Traefik v3.6.7,
so we don't need to override their values.

Ref: https://doc.traefik.io/traefik/v3.6/migrate/v3/#v367

Closes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/4835

Provoked by https://github.com/element-hq/synapse/pull/19281
which landed in Synapse v1.145.0, but we pull in a few other routes
that I noticed to be missing.