feat(worker): real per-job cancellation for the background job pool

CancelJob existed but was dead code: it only flipped a status in the
in-memory results map - it never cancelled anything, and processJob's
unconditional completion write overwrote the marker anyway.

- derive a per-job context in processJob (parent is the handler ctx
  for manual scans, Background for auto-scans) and register its
  cancel func under the job id; previously every manual scan shared
  the handler's process-lifetime context, so no targeted cancel was
  possible without killing everything
- skip execution at dequeue for jobs cancelled while still queued
- record a pending JobResult on BOTH enqueue paths (Enqueue used by
  auto-scans, EnqueueJob used by the API): queued jobs were invisible
  to status polling (404 until a worker picked them up) and could not
  be cancelled before execution started
- cancelled jobs keep their last real progress instead of a fake 100%
  and drop the meaningless "context canceled" error text
- broadcast the new scan_cancelled websocket event for cancelled scan
  jobs instead of scan_complete, so clients reset their scanning
  indicators rather than reading the abort as a finished scan
- sentinel errors (ErrJobNotFound, ErrJobNotCancellable) so HTTP
  handlers map outcomes to 404/409 without string matching

Unit tests (worker_cancel_test.go) pin: queued-cancel never executes,
running-cancel invokes the context, error mapping, pending recording
on both enqueue paths.

internal/sync/websocket.go

@@ -20,6 +20,7 @@ const (
 	MessageTypeScanProgress     = "scan_progress"
 	MessageTypeScanComplete     = "scan_complete"
 	MessageTypeScanError        = "scan_error"
+	MessageTypeScanCancelled    = "scan_cancelled"
 )
 
 // BroadcastMessage represents a message to broadcast to connected clients