callback args hardening.

[T4rk1n]

Fix background callbacks trusting the client-supplied job/oldJob/cancelJob and cacheKey query parameters verbatim, which let an unauthenticated client terminate an arbitrary process (with DiskcacheManager, by sending its PID) or read and delete arbitrary result-cache entries. The cacheKey/job handles are now HMAC-signed by the server and bound to a per-page-load token, so forged or replayed values are rejected. Handles stay opaque to the renderer, so no app or callback code changes are required.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[camdecoster]

This seems fine, but I think @KoolADE85 should give it look before you merge.