chore: sunset the StaticMirroringWorker in favour of negentropy sync #923

Is your feature request related to a problem? Please describe.
StaticMirroringWorker (src/app/static-mirroring-worker.ts, 417 lines) mirrors events from other relays through its own admission check, separate from the one EventMessageHandler uses. Rules added to the write path have to be repeated there: for example, with subscription plans enabled, StaticMirroringWorker.isUserAdmitted never reads the subscription, so mirrored writes skip expiry (raised on #909). It also has no unit tests (#904).

Describe the solution you'd like
Retire the worker, since NIP-77 (Negentropy) sync and other ways of syncing with relays now cover what it is used for:

• Remove StaticMirroringWorker, its factory and its start-up wiring.
• Remove or deprecate the mirroring.static[] settings (including skipAdmissionCheck), the settings docs and the admin console parts that depend on them, with a changeset noting the removal.
• Document how to sync with another relay without it.

Additional context

• Decision from the maintainer on feat(payments): add subscription lifecycle, renewal invoices and plan-based write access #909: sunset the worker rather than patch the subscription check into it.