Record a dashboard view (telemetry beacon).

Fire-and-forget telemetry beacon (DVT-1215): records that this dashboard was actually viewed, including a cache-served render that leaves no query_executions row (the warehouse-pushdown path only records a row when the engine is actually hit). Requires dashboard read-level access — the same access any viewer already has. Meant to be called via navigator.sendBeacon; the body is optional and a malformed pageId is silently dropped rather than rejected — this endpoint always responds 204 once the caller is authorized, and the write itself happens asynchronously off the request path (an error writing the row is logged and dropped, never surfaced to the caller). A second view from the same actor for the same dashboard + page within 60 seconds is deduped server-side (not double-counted).