Issue #3605912: orchestra_audit_trail config schema missing events.task.cancelled

Config Inspector reported a "missing schema" error for orchestra_audit_trail: events.task.cancelled was "not a supported key".

The task channel emits a "cancelled" audit event (the inbox cancels a work item when its token is cancelled), and the settings form offers a "Cancelled" toggle for the task channel, so saving the form writes events.task.cancelled to the active config. But the config schema, and the install defaults, only declared the task actions up to "reoffered", so the saved key failed schema validation.

Fix

• Add cancelled to the task events in the config schema and in the install defaults (default on, matching the instance and token cancelled events).
• Add a regression test that submits the settings form: it persists one boolean per (channel, action) the form offers, so the save validates them all against the schema. With strict schema checking on, a key missing from the schema throws on save.

Why it escaped CI: the strict config-schema check validates only config installed or saved during a test. The install default never carried the key (it stopped at "reoffered"), and no test saved the settings form, which is the only writer of events.task.cancelled. So the invalid key was never produced in a test; only the live UI save produced it, which is why Config Inspector caught it and CI did not. The new test closes that gap.