Jumoo.Processing: cancellation tokens, pipeline pruning, CI parity, and v17.4.0-rc1

Added CancellationToken overloads to IPipelineService.Process / IProcessingService.ProcessAsync (existing overloads obsoleted, removed in v20) so an aborted source request stops the target mid-step instead of running a full export to completion
Indexed Jumoo_Pipelines.Key and added a recurring job to prune Completed/Failed rows — the table only ever grew before
Ported v18/main's CI/CD pipeline and repo standards to v17/main: GitHub Actions (PR build, package build, tagged release), central package management with locked restores, two new test projects (41 tests total), npm workspaces for the backoffice assets, and the usual repo files (LICENSE, SECURITY, CODE_OF_CONDUCT, .editorconfig, etc.)
Released v17.4.0-rc1 to NuGet — first real proof the ported pipeline works end to end