MSC4140: allow auth on management endpoints for delayed events

[AndrewFerr]

This is to allow authed requests to have their own ratelimit quotas.

MSC4140

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[Copilot]

Pull request overview

Enables authenticated requests to delayed event management endpoints (MSC4140) to be rate-limited based on the authenticated user rather than the source IP, so authenticated clients can have their own quotas distinct from unauthenticated/IP-based traffic.

Changes:

• Add a shared _mgmt_ratelimit helper in DelayedEventsHandler that switches ratelimiting keys between authenticated user and unauthenticated source IP.
• Expand delayed event management ratelimit tests to cover the “authenticated bypasses IP ratelimit” behavior and per-user override disabling ratelimits.
• Update configuration schema/docs wording to reflect the new ratelimiting behavior, and add a Towncrier feature fragment.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/rest/client/test_delayed_events.py	Updates ratelimit tests to cover authenticated-vs-unauthenticated quotas and user ratelimit overrides.
synapse/handlers/delayed_events.py	Implements auth-aware ratelimiting for delayed event cancel/restart/send; simplifies user listing ratelimit keying.
schema/synapse-config.schema.yaml	Updates config schema description for rc_delayed_event_mgmt to match new behavior.
docs/usage/configuration/config_documentation.md	Updates user-facing config docs description for delayed event management ratelimiting.
changelog.d/19794.feature	Adds Towncrier newsfragment describing the new authenticated behavior.

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[anoadragon453]

Just some minor wording changes. Otherwise this LGTM!